(CNN) -- A South Korean navy ship sank in the Yellow Sea near North Korea late Friday, and the navy shot at an unidentified ship toward the north, according to reports quoting South Korean government officials. Yon-hap 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 Cheonan went down at 9:45 p.m. near the island, but the cause of the incident was not immediately known, the officials said. South Korean President Lee Myung-bak ordered a "quick and thorough" investigation Saturday into what caused the ship to sink, keeping in mind "all possibilities," his office said, according to Yonhap.
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NEW: South Korean president orders "quick and thorough" investigation
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Korean media reports 58 of 104 crew members rescued
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Navy fired on an unidentified ship toward the north, Yonhap reports
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Officials say an explosion occurred at ship's stern but cause remains unclear
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Seoul government says it isn't clear if a North Korean vessel was involved in the incident
A rescue operation was under way, with the @placeholder telling Yonhap it had rescued 58 crew members but it feared others may be dead.