The elusive quest for peace in Syria is now crippled with recent setbacks, as a U.N. observer mission has suspended operations and attempts to rescue civilians trapped in violence have proved futile.

Chaos erupted once again Monday as regime forces shelled the southern town of Tafas, an opposition group said, prompting calls for residents to hunker down in lower floors of houses. The attack came after more than 80 tanks infiltrated the town in Daraa province, the Local Coordination Committees of Syria said.

Elsewhere in Daraa, warplanes hovered at a low altitude over the city of Daeel as powerful explosion rocked the city, the LCC said.

Violence in the country has escalated in recent days, exacerbating an already risky situation for the about 300 monitors, said Gen. Robert Mood, who heads the U.N. monitoring mission in Syria.

"Civilians continue to be trapped by the escalating violence in Syria," Mood said in a statement Sunday. "In Homs, attempts to extract civilians from the line of fire over the past week have been unsuccessful."

Fifteen months into the Syrian crisis and with no end in sight, a prominent opposition body has elected Kurdish activist Abdul Baset Sieda as its new president in what could reinvigorate a political arm of the uprising.

Dozens of countries have recognized the SNC as a legitimate representative of the Syrian opposition, though many members of the group's leadership are expatriates.

But Syrian President Bashar al-Assad - who has received global disdain for lethally cracking down on dissidents seeking his ouster - has said he will not deal with opposition members influenced from the outside.

Meanwhile, renewed attacks on the anti-government bastion of Homs killed at least three people on Sunday, including citizen journalist Khaled Bakr, opposition activists said.

According to the LCC, random shelling tormented the Homs neighborhood of Talbiseh early Sunday, pausing every four minutes only to continue with artillery and mortars.

Human Rights Watch is calling for a boycott of new arms contracts with companies supplying the Syrian regime as the nearly 15-month government crackdown forced the country to the brink of a civil war.

The call comes after U.N. envoy Kofi Annan said Syria is "at a turning point" and that "the specter of all-out civil war, with a worry sectarian dimension, grows by the day."

Human Rights Watch on Sunday singled out Rosoboronexport, a Russian state-owned weapons supplier, for sending arms to the Syrian regime.

"The (U.N.) Security Council should impose a mandatory international arms embargo on Syria, and Russia and China should not block it," the group's executive director, Kenneth Roth, said in a statement. He accused the Syrian regime of crimes against humanity and said "other governments and companies around the world should use whatever leverage they have to stop further arms supplies that could contribute to these crimes."

Rosoboronexport did not immediately respond to the rights group's allegations.

U.N.-Arab League special envoy Kofi Annan has expressed dismay over reports that the Syrian government resumes brutal attacks on cities once U.N. monitors leave - the latest sign that diplomatic efforts are failing to quash the bloodshed in the country.

"I am particularly alarmed by reports that government troops entered Hama (Monday) after observers departed, firing automatic weapons and killing a significant number of people," Kofi Annan told U.N. Security Council members Tuesday. "If confirmed, this is totally unacceptable and reprehensible."

Thirty observers are expected in the country by Monday, and 100 within a month, said U.S. Ambassador to the U.N. Susan Rice, citing comments by Herve Ladsous of U.N.'s Department of Peacekeeping Operations.

"Mr. Ladsous reported that the Syrian government has refused at least one observer based on his nationality, and that Syrian authorities have stated they will not accept (monitoring mission) staff members from any nations that are members of the 'Friends of Democratic Syria,'" Rice said Tuesday. "He underscored that from the U.N. point of view this is entirely unacceptable."

The first members of a U.N. observer team will begin their work in Syria on Monday, tasked with monitoring a tenuous cease-fire that is showing signs of collapse.

"They will be liaising with the Syrian government, security forces and the opposition members to establish the monitoring process across the country," said Kieran Dwyer, a spokesman for peacekeeping missions at the United Nations.

But since the deadline for the cease-fire passed on Thursday morning, reports of deadly violence at the hands of the regime continued.

At least 23 people died across Syria on Sunday, including 11 in the besieged city of Homs, opposition activists said. Government helicopters pummeled Homs from the sky, said the Local Coordination Committees of Syria, a network of opposition activists.

But the state-run Syrian Arab News Agency blamed "armed terrorists" for violating the cease-fire, quoting an unnamed military official's assessment that the groups "have hysterically escalated their aggressions on the army, the law enforcement forces and the civilians."

Government helicopters pounded the besieged city of Homs from the sky, opposition activists said Sunday, three days after a so-called cease-fire in Syria.

In addition, "one bomb is being shelled every 10 minutes from the military academy, aiming at al-Wair neighborhood in Homs," said the Local Coordination Committees of Syria, a network of opposition activists.

At least nine people died across Syria on Sunday, including six in Homs, said the opposition Syrian Network for Human Rights. Two died in Aleppo a day after they were injured when regime forces opened fire at a funeral procession, the group said.

The latest reports of violence came three days after a deadline to stop the bloodshed and one day after the U.N. Security Council voted unanimously  to authorize unarmed observers to travel to Syria to monitor the shaky cease-fire.

A deadline for Syrian troops to withdraw from cities came and went Tuesday morning, but opposition activists say regime's onslaught rages on.

One activist in the besieged city of Homs said fresh shelling rained on two neighborhoods late Tuesday morning.

"Tanks hitting those areas remain in al-Qusoor neighborhood. So there is not a tank that pulled out from there," said the opposition activist, identified only as Omar for safety reasons.

Hours before the deadline, reports of mounting carnage left some world leaders doubtful of the regime's promises.

"Should the Syrian government yet again refuse to implement its commitments, make promises and then break them and continue and escalate the killing, then I think it will be clear to all that there isn't yet prospect for diplomatic solution," Susan Rice, U.S. ambassador to the United Nations, said Monday. "We still hope that that's possible, we still want to give that a final chance, but I don't think we, or anybody else, are particularly optimistic."

The Syrian military clashed with defectors in the capital of Damascus on Saturday as regime forces bombarded other towns with heavy gunfire despite the United Nations' call for a cease-fire, opposition activists said.

Gunfire between both sides erupted in the Homs neighborhoods of Baba Amr and Jouret Al-Arayes, the opposition Local Coordination Committees of Syria said. Early-morning shelling also targeted other various areas in Daraa, Idlib, Hama and the Damascus countryside, opposition activists said.

The latest violence comes after a U.N. special envoy ordered President Bashar al-Assad to implement a peace plan and not await concessions from the opposition.

"The government must stop first and then discuss a cessation of hostilities with the other side and with the mediator," Ahmad Fawzi, a spokesman for special envoy Kofi Annan said Friday. "We expect him to implement this plan immediately."

A day after news broke that Syrian President Bashar al-Assad accepted a plan to forge peace, fresh clashes broke out once again across Syria on Wednesday, opposition activists said.

Three members of the Syrian security forces were killed and four defecting soldiers were injured from intense clashes at an entrance to the western city of Rastan, which security forces have been trying to storm, said the Syrian Observatory for Human Rights, an opposition group.

And fierce fighting erupted between the Syrian security forces and a group of defectors in the southern Daraa province, the observatory said. The group said clashes began after an army soldier threatened residents in the town of Busr el-Harir to hand over the group of defectors or face a military operation.

Opposition activists have said Busr el-Harir has endured shortages of food and medicine since the Syrian army surrounded the town weeks ago.

The reports of violence come after al-Assad accepted a plan laid out by U.N.-Arab League special envoy Kofi Annan to help end the bloodshed.

U.N.-Arab League special envoy Kofi Annan arrived in Beijing on Tuesday to rally support in helping end the bloodshed in Syria.

Annan's visit to China comes after a stop in Russia, the other country that has stymied U.N. Security Council attempts to condemn the Syrian regime.

During his two-day trip to China, Annan will meet with foreign ministry officials, China's state-run Xinhua news agency said.

Both China and Russia have said they want the violence to stop but argued that draft resolutions by Security Council peers were not evenhanded. Both countries have major trade ties with Syria, but have said they are not trying to protect a regime.

Disjointed factions of the Syrian opposition are meeting Monday in Istanbul in an effort to form a unified voice ahead of the next week's "Friends of Syria" conference.

Observers of the Syrian crisis have bemoaned a lack of unity among the various Syrian opposition groups as they struggle to topple President Bashar al-Assad's rule.

The goal of Monday's conference is "for all forces and parties of the opposition not to be a union, but at least to have a united purpose," said Amar Qurabi, leader of the National Change Current opposition group.

At least nine people were killed across Syria on Monday, the opposition Syrian Network for Human Rights said. The dead include four men gunned down in Homs when residents tried to retrieve two bodies, the group said.

The United States and Turkey are in "full agreement" on the next steps on Syria as world leaders continue trying to stop the year-long massacre that has killed thousands.

U.S. President Barack Obama met with Turkish Prime Minister Recep Tayyip Erdogan on Sunday in South Korea, ahead of this week's nuclear summit in Seoul.

The meeting was aimed at "syncing up" with Turkey - a former Syrian ally - before Istanbul hosts the next "Friends of Syria" meeting on April 1, a senior Obama administration official said. A goal of that conference is to exert pressure on Syrian President Bashar al-Assad's regime for its brutal crackdown on civilians.

Obama and Erdogan expect the upcoming conference to enhance ways to provide "non-lethal" aid to the opponents of the Syrian regime, such as medical supplies and communications equipment, said Ben Rhodes, a U.S. deputy national security advisor.

"We cannot be spectators," Erdogan said Sunday.