[Updated, 8:07 p.m.] Read the full CNN.com story
[Updated at 1:07 p.m.] The Yonhap News Agency is reporting that the South Korean navy ship has sunk, according to government officials.
[Updated at 12:26 a.m.] South Korea's president has told his military to focus on rescuing sailors from one of their navy ships sinking in the Yellow Sea near North Korea, aids told the Yonhap News Agency.
Yonhap reported 58 sailors had already been rescued. Navy vessels and helicopters were rescuing crew members, some of whom reportedly jumped into the sea after the blast, an official told South Korea's Korean Broadcasting System. The ship, the Cheonan, was patrolling southwest of the island around 9:45 p.m. when the explosion took place, a military official told KBS.
The official said the Sockcho, another South Korean navy vessel patrolling nearby, fired at unidentified ships north of the area. U.S. military officials told CNN a South Korean vessel has sunk.
The South Korean government issued a statement saying the reason for the incident remains unclear.
"For now, it is not certain whether North Korea is related" to the incident, South Korean spokeswoman Kim Eun-hye told Yonhap.
While the rescues continue, officials were holding an emergency meeting of ministers handling security-related matters, but the focus remained on rescuing South Koreans on the ship, the Yonhap News Agency reported.
"Finding the truth [behind the incident] is important, but saving our soldiers is more important," the president was quoted as saying, according to Yonhap.
North Korea has said recently it is bulking up its defenses in response to recent joint South Korean-U.S. military drills, Yonhap reported. The news agency also quoted South Korean military officials saying North Korea conducted dozens of artillery firing drills Friday.
[Posted at 11:01 a.m.] A South Korean navy ship was sinking in the Yellow Sea near North Korea early Saturday, and the navy was shooting at an unidentified ship toward the north, Yonhap News Agency reported.
It was initially unclear whether the two incidents were related.
Yonhap quoted Navy officials saying Friday that a ship carrying 104 crew members was sinking off the Seoul-controlled island of Baengnyeong.
The 1,500-ton ship was sinking between 9 and 10 p.m. near the island, but the cause of the incident was not immediately known, officials said.
A rescue operation was under way. There were no immediate reports of casualties, but some sort of explosion occurred in the rear of the ship, officials told Yonhap.
As a result of the report, South Korean government officials held an emergency meeting of ministers handling security-related matters, officials told the news agency.
There were no further details about the reported shooting.