(CNN) -- A South Korean navy ship sank in the Yellow Sea near North Korea late Friday, and the navy fired on an unidentified ship toward the north, according to reports quoting South Korean government officials.

Yonhap News Agency quoted navy officials saying Friday that a ship carrying 104 crew members sank off the Seoul-controlled island of Baengnyeong in a flashpoint maritime border area between the Koreas.

The 1,500-ton corvette Cheonan went down at 9:45 p.m. near the island, but the cause was not immediately known, the officials said.

A rescue operation was under way, with the navy telling Yonhap it had rescued 58 crew members but that others may be dead.

The South Korean government issued a statement saying it wasn't ruling out some sort of military engagement. An explosion had occurred in the rear of the ship, leaving a large below the water line, officials told Yonhap.

A U.S. State Department spokesman said Friday that there was no evidence North Korea was behind the incident.

"Let's not jump to conclusions here," said the spokesman, Philip J. Crowley. "I'm not aware of any evidence to that effect, but I think the authoritative source here would be the South Korean government."

Yonhap quoted South Korean naval officials as saying a South Korean vessel later had fired on a ship toward the north.

However, South Korean government officials said it isn't certain whether North Korea was involved. Yonhap said residents reported hearing gunfire for about 10 minutes.

As a result of the incident, South Korean government officials held an emergency meeting of ministers handling security, officials told Yonhap.

Aides to South Korean President Lee Myung-bak said the priority is rescuing crew members.

South Korea's Korean Broadcasting System said navy vessels and helicopters were rescuing crew members, some of whom reportedly jumped into the sea after the blast.

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.