(Reuters) - Two North Korean vessels crossed into the South's waters off the peninsula's tense west coast before returning when South Korea fired warning shots, military officers said on Sunday.
The area has been a site of military standoffs and is near where a South Korean navy ship sank in March after apparently being struck by a torpedo, killing 46 sailors.
South Korea has not officially blamed the North for the attack but officials have made little secret of their belief Pyongyang was behind the attack. An international team of investigators are expected to release their findings this week.
"Two patrol boats crossed on two separate occasions and warning shots were fired," said an officer at the South's Office of the Joint Chiefs of Staff.
The apparent maritime border violation by the North's vessels come amid a deepening chill in relations between the rival Koreas, which remain technically at war under a truce that ended fighting in the 1950-53 Korean War.
South Korea's belief in the North's involvement in the sinking of its navy corvette Cheonan has been a source of friction between Seoul and Beijing, and Chinese Foreign Minister Yang Jiechi tried to cool South Korea's exasperation when top diplomats from the two countries and Japan met this weekend.
In contrast to the aloof response shown by Yang the day before, Japan's Katsuya Okada was more sympathetic to the South when he met Foreign Minister Yu Myung-hwan on Sunday in Gyeongju, the scenic ancient capital of Shilla, a Buddhist kingdom that ruled the peninsula in the first millennium.
"(Okada) expressed his high respect for our government's efforts to get to the cause through scientific and objective means under what can only be a truly difficult situation," a South Korean official said after their meeting.
"The two sides agreed (on) close cooperation in the response following the investigation into the Cheonan incident."
The Chinese foreign ministry on Saturday called for "calm and restraint" in dealing with the Cheonan sinking.
South Korea knows it cannot launch a retaliatory strike against the North without risking greater conflict and undermining its own economy, just recovering from the global financial slump.
But it does want international punishment of the North. That would likely mean even tougher sanctions by the United Nations, which would need China's support to take effect.
There is media speculation in South Korea that Secretary of State Hillary Clinton will visit Seoul later this month in a show of support over the sinking.
There was little formal discussions at the three-way meeting on how to resume the stalled negotiations with North Korea on compensating it in return for concrete steps to end its nuclear arms program.
South Korea and Japan said they would not sit at the table with the North if Pyongyang is found to be the culprit in the navy ship sinking. China hosts those talks, which also involve the United States and Russia and have been stalled since 2008.
(Editing by Jerry Norton)
