South Korea's navy fired warning shots to drive away North Korean patrol boats from the disputed inter-Korean sea border, amid tension over the sinking of a Seoul warship, officials said Sunday.

The warning shots were fired late Saturday when two North Korean patrol boats violated the Northern Limit Line (NLL) border and strayed into South Korean waters, the South's Joint Chiefs of Staff office said.

A spokesman for the Joint Chiefs of Staff told AFP the North's patrol boats retreated without responding to the fire. No casualties occurred.

It is the first time that shots have been fired at the tense sea border since the mystery sinking of a South Korean warship on March 26.

Suspicions have since been growing that a North Korean torpedo downed the warship, killing 46 South Korean sailors.

The sinking incident was discussed at the weekend talks between foreign ministers from South Korea, Japan, and China in the southern city of Gyeongju.

Minister Yang Jiechi of China, the North's sole major ally and economic lifeline, joined his South Korean and Japanese counterparts to express sympathy over the heavy loss of life, according to a joint statement.

"We expressed our condolences for the loss of many lives due to the sinking of the (South Korean) navy ship Cheonan on March 26," the statement said.

A multinational investigation is to report by Thursday, and Seoul is weighing its diplomatic and economic options if the North -- which denies involvement -- is found to have sunk the corvette.

China is the North's sole major ally and its economic lifeline. As a veto-wielding member its backing would be crucial if the South takes the matter to the United Nations Security Council.

Baek Seung-Joo, an analyst at the Korea Institute for Defense Analyses in Seoul, told AFP the North's latest breach of the sea border aimed to cement its territorial claims ahead of the annual season to fish crabs.

"The North is saying that tension caused by the Cheonan incident does not affect its policy of seeking to nullify the NLL in the Yellow Sea," Baek said. "Especially when the June crab-catching season is just around the corner."

The Joint Chiefs of Staff office said one North Korean patrol boat crossed into South Korean waters near Yeonpyeong island at 10:13 pm (1313 GMT) Saturday and retreated after a radio warning from the South.

A second North Korean patrol boat violated the border in the same area 47 minutes later, ignored a radio warning from the South and sailed north only after warning shots were fired from the South, it said.

The area was the scene of deadly naval clashes in 1999 and 2002, and of a firefight last November which set a North Korean patrol boat ablaze.

The North has never recognised the NLL border drawn by the US-led United Nations Command after the 1950-1953 Korean war ended in an armistice. But the South has maintained it as a de facto inter-Korean border.

The South's 655,000-strong military, backed up by 28,500 US troops, still faces off against the North's 1.2 million-member military.

Seoul's Defence Minister Kim Tae-Young last week confirmed that investigators had found traces of RDX explosive, which is widely used in torpedoes, on the wreckage and the seabed.

The North Sunday threatened to block South Koreans from crossing the land border if Seoul does not prevent South Korean activists from launching leaflets into the communist state, the North's official news agency was quoted saying.

earlier related report
Canada sends experts to probe sinking of S.Korean warship
Ottawa (AFP) May 16, 2010 - Canada dispatched Sunday three naval experts to help investigate the sinking of a South Korean warship that has raised tensions between Seoul and Pyongyang.

Canadian Foreign Minister Lawrence Cannon said Seoul had invited the experts to join a multinational probe into the March 26 sinking.

There are suspicions that a North Korean torpedo downed the warship near the disputed inter-Korean border with the loss of 46 lives.

A multi-national investigation is to report by next Thursday and Seoul is weighing its diplomatic and economic options if the North -- which denies involvement -- is found to have sunk the corvette.

"Canada is strongly committed to the security and stability of the Korean Peninsula," Cannon said in a statement.

"Canada-Korea bilateral relations are firmly grounded in our history of strong political and economic partnership and cooperation. We are pleased to provide assistance to a key partner in the region."

Both Canada and South Korea are hosting G20 summits this year.