(CNN) -- South Korean rescuers were searching for missing sailors Saturday after a navy ship sank in tense Yellow Sea waters off the coast of North Korea.
The 1,200-ton patrol ship Cheonan was on routine patrol when it sank Friday at 9:45 p.m. A cause has yet to be determined but the Yonhap News Agency quoted military officials as saying that an unidentified explosion punched a hole in the bottom of the ship.
The vessel was carrying 104 sailors, 58 of whom have been rescued. Divers continued the search for 46 missing sailors in unseasonably cold temperatures and treacherous waters.
A photograph released to media showed a section of the ship's hull still above water, raising hopes that the sailors could be alive.
Tensions simmered after the Cheonan went down off Baengnyeong, a Seoul-administered island in a flashpoint maritime border area between the Koreas.
South Korean President Lee Myung-bak has ordered a "quick and thorough" investigation into what caused the ship to sink, keeping in mind "all possibilities," his office said, according to Yonhap.
He convened a meeting of national security-related ministers immediately after the incident occurred, and a second one on Saturday morning.
Given Baengyeong island's proximity to North Korea, North Korean involvement was feared, but South Korean officials have been playing down that scenario. And 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."
Baengnyeong residents had reported hearing gunfire at sea shortly after the Cheonan sank. South Korean officials later confirmed that on of their ships had fired on a radar contact that turned out to be a flock of birds.
Baengnyeong lies on the Northern Limit Line (NLL), the maritime border between the two states which North Korea disputes and which covers rich crab fishing grounds.
The NLL was the scene of fatal naval skirmishes in 1999 and 2002. The two Koreas also exchanged naval gunfire in 2004 and 2009.
In recent months, North Korea has been firing coastal artillery into the waters near the island, and shells had been fired earlier Friday, according to news reports.
Journalist Andrew Salmon contributed to this report.