[Updated at 7:46 p.m. ET] Although an Algerian military raid against the hostage-takers wrapped up on Thursday, a senior U.S. official stresses that more military operations could be coming.
"There are still hostages, and there are still terrorists," the official said, according to CNN's Elise Labott. "So tomorrow is another day."
[Updated at 5:51 p.m. ET] British Prime Minister David Cameron has warned his compatriots to prepare for "bad news ahead" related to kidnapping of dozens of hostages at a BP gas plant in Algeria.
"It is a fluid situation, it is ongoing," Cameron told the Reuters news agency. "But I think we should be prepared for the possibility of further bad news, very difficult news, in this extremely difficult situation."
[Updated at 5:49 p.m. ET] Some American hostages have been freed, but others are still unaccounted for, U.S. officials tell CNN's Elise Labott. The officials did not provide any further details.
[Updated at 3:36 p.m. ET] U.S. Secretary of State Hillary Clinton has asked her staff to review security for American diplomats, businesses and citizens in the entire Maghreb and North Africa region, in response to the hostage-taking in Algeria, State Department spokeswoman Victoria Nuland said.
[Updated at 3:18 p.m. ET] The Algerian military operation to free hostages at the gas plant has ended, the official news agency of Algeria, APS, reported.
[Updated at 2:40 p.m. ET] U.S. Secretary of State Hillary Clinton says will speak with Algeriaâ€™s foreign minister this afternoon about the hostage situation. She last spoke to him on Wednesday.
â€śThe situation is very fluid,â€ť Clinton told reporters at the State Department. â€śThe security of our Americans who are held hostage is our highest priority.â€ť
[Updated at 2:05 p.m. ET] The United Nations has now weighed in on the Algerian hostage crisis. "With regard to Algeria, I would simply say, we are aware of the reports coming out of the region. I don't think anyone has a clear picture yet of what has unfolded and what I can say now is that we utterly condemn any kidnapping or hostage taking - and that our thoughts are for the safety of those being held and for their families," U.N. spokesman Martin Nesirky said.
[Updated at 1:30 p.m. ET] An Algerian military operation that freed some hostages and left an unspecified number of casualties today still is happening, the Algerian communications minister said on Algerian state TV.
The minister, Mohamed Said, said the military was able to free a "large number" of hostages, but that casualties also happened.
[Updated at 12:50 p.m. ET] The hostage-takers have AK-47 firearms and put explosive-laden vests on some of the hostages, a U.S. State Department official said.
It is not clear whether the hostage-takers wore the suicide vests when they took the oil facility Wednesday, but they did put them on some of the hostages, another U.S. official said.
[Updated at 12:47 p.m. ET] Earlier, we told you that an Irish hostage was free, according to the Irish government. Now comes reaction from the Irish prime minister:
"The kidnap ordeal of Belfast man Stephen McFaul has ended," Irish prime minister Enda Kenny said. "I am greatly relieved to hear that Stephen is safe and well. I believe he has already spoken to his family in Belfast and I wish him a safe return home to his loved ones."
It is not clear if he was freed by an Algerian military operation. Algerian media reported earlier today that the Algerian army freed four hostages - two Britons, a Kenyan and a French citizen - and that the action also resulted in a number of casualties.
[Updated at 12:22 p.m. ET] White House spokesman Jay Carney says the United States believes Americans are among the hostages, confirming what U.S. officials said yesterday. But he hasn't offered a number, and he says the United States is "seeking clarity from the government of Algeria."
"It is our understanding that there are Americans involved, but I would say a couple of things. One, we condemn in the strongest terms a terrorist attack on BP personnel and facilities in Algeria and we are closely monitoring the situation," Carney said at Thursday's White House press briefing. "We are in contact with Algerian authorities and our international partners as well as (the) BP security office in London.
"Unfortunately, the best information that we have at this time, as I said, indicates that U.S. citizens are among the hostages. But we do not have, at this point, more details to provide to you. We are certainly concerned about reports of loss of life and we are seeking clarity from the government of Algeria."
[Updated at 11:10 a.m. ET] A U.S. Predator drone has flown over the gas plant to gather intelligence on the site, a U.S. official tells Barbara Starr.
[Updated at 10:46 a.m. ET] An Irish citizen who was among the hostages in now free and has made brief contact with his family, the Irish Department of Foreign Affairs tells CNN.
Information on how the Irish hostage left his captors wasn't immediately available. The news comes as energy company BP says that the Algerian army is trying to take control of the gas installation where hostages were being held, and just minutes after Algeria's state media reported that four other hostages - two Britons, a Kenyan and a French citizen - were freed in an Algerian army operation.
The Algerian media report also said there were casualties from the operation, but the number is not yet known. CNN could not independently confirm the report.
CNN affiliate BFM-TV reported the French citizen is a nurse who worked on the site. The number of hostages that were taken isn't clear. Two U.S. officials said Wednesday that the number of American hostages could be as few as three.
[Updated at 10:31 a.m. ET] The Algerian army is "attempting to take control" of the gas installation where foreign hostages are being held, BP says, citing British and Algerian authorities.
BP said it cannot confirm any reports of hostages being killed, released or escaping, according to CNN's Carol Jordan.
Algeria's state media reported earlier Thursday that four hostages - two Britons, a Kenyan and a French citizen - were freed in an Algerian army operation. It also reported earlier that all Algerian nationals who had been held hostage were free: some had fled, while others were released.
All the hostages still detained are foreigners, Algerian Interior Minister Diho Weld Qabliyeh said. The number of hostages is unclear.
The U.S. State Department has said it is still working to determine how many citizens were involved. American hostages could be as few as three, two U.S. officials said Wednesday.
[Updated at 10:14 a.m. ET] Here's the latest on four hostages who reportedly were freed in an Algerian army operation: They are two Britons, a Kenyan and a French citizen, according to Algerian state news. CNN could not independently confirm the report.
[Updated at 9:53 a.m. ET] Four of the foreigners taken hostage by Islamist militants at an Algerian gas plant have been freed in an operation by the Algerian army, the Algerian state-run APS news agency reported Thursday.
[Initial post, 9:50 a.m. ET] "Algerian authorities have confirmed that there is an ongoing operation," as foreigners are held hostage at a gas installation in Algeria, the British Foreign Office tells CNN.
The BP gas field is 60 kilometers (40 miles) west of the Libyan border and 1,300 kilometers from the Algerian capital, Algiers.
The militants said they carried out the operation Wednesday in Algeria because it allowed French forces to use its air space in attacking Islamist militants in Mali.
The fallout escalated after rebels kidnapped Westerners, dragging governments beyond Africa into a daunting insurgency.
"The situation on site remains unresolved and fragile," BP said in a statement. "Armed groups still occupy the site and hold a number of site personnel. "
During the initial attack on the gas plant, kidnappers killed two people - an Algerian and a British national, the Algerian Press Service reported.