North Korea accused South Korea's navy Tuesday of trespassing in its waters and threatened military action, further raising tensions sparked by the sinking of one of Seoul's warships in March.

The South has announced a series of reprisals including a trade ban after a multinational investigation concluded last week that a North Korean submarine had torpedoed the Cheonan on March 26.

The communist North denies involvement in the sinking, which claimed 46 lives and brought it widespread international condemnation. It threatens full-scale war if there is any attempt to punish it.

The North's military made its complaint Tuesday in a message to the South's armed forces, Pyongyang's official Korean Central News Agency reported.

Over 10 days ending Monday, it said, dozens of the South's naval ships had intruded into the North's waters.

"This is a deliberate provocation aimed to spark off another military conflict in the West (Yellow) Sea of Korea and thus push to a war phase the present North-South relations that reached the lowest ebb," KCNA quoted the message as saying.

If the intrusions continued, the North "will put into force practical military measures to defend its waters...and the South side will be held fully accountable for all the ensuing consequences".

The defence ministry denied any of its ships had crossed the border known as the Northern Limit Line, which the North refuses to accept.

The flashpoint frontier was the scene of deadly naval clashes in 1999 and 2002 and of a firefight last November that left a North Korean patrol boat in flames. Seoul military chiefs believe the Cheonan was sunk in revenge.

The South has ruled out a military counter-attack but is seeking diplomatic support to refer the sinking to the United Nations Security Council.

In an apparent show of strength, the defence ministry said the South's navy would stage an anti-submarine drill in the Yellow Sea Thursday. A spokesman would not confirm a Yonhap news agency report that 10 ships would be involved.

The military also said a destroyer is stationed in the Jeju Strait off the south coast to stop the North's cargo ships using it. The South shut its sea lanes to the North as part of reprisals announced Monday.

Financial markets across Asia responded nervously to the escalating crisis, with one unconfirmed report from a group of North Korean defectors claiming that the North had placed its armed forces on combat alert.

South Korea pressed a visiting Chinese official Tuesday to support international efforts to punish the North.

"The Chinese position still appears to be unclear," a foreign ministry official said after Wu Dawei held talks with Foreign Minister Yu Myung-Hwan and chief nuclear envoy Wi Sung-Lac.

"What the Chinese side mainly emphasised was stability and peace on the Korean peninsula."

China -- which could veto any UN move for new sanctions -- has not publicly blamed its ally the North for the sinking, despite pressure from visiting US Secretary of State Hillary Clinton to get tougher.

"The two sides believe that ensuring peace stability in east Asia and the Korean peninsula is critical," State Councillor Dai Bingguo said in Beijing at a joint press appearance with key US officials.

"Relevant parties should proceed on the basis of safeguarding the overall interest of peace and stability in the region and calmly and appropriately handle the issue and avoid escalation of the situation."

Clinton said the two sides share the objective of peace and stability on the Korean peninsula. "Now we must work together again to address the serious challenge provoked by the sinking of the South Korean ship."

The United States has strongly backed Seoul's punitive measures and announced it would soon hold anti-submarine and other naval exercises with it.

The North says the South's "puppet" authorities have faked evidence of its involvement as part of a plot to ignite conflict.

"The servicepersons and members of the Worker-Peasant Red Guards...are determined to strongly react to their gangster-like provocation," KCNA reported.

The Seoul-based defector group, North Korea Intellectual Solidarity, said the combat alert order was issued by leader Kim Jong-Il the day the investigation team reported last Thursday.

earlier related report
US lawmakers back S. Korea in feud with N. Korea
Washington (AFP) May 24, 2010 - US lawmakers on Tuesday overwhelmingly passed a resolution condemning North Korea for sinking a South Korean warship and demanding the secretive regime in Pyongyang apologize for the attack.

The House of Representatives voted 411-3, with 17 not casting ballots, to approve the symbolic resolution, which also expresses US sympathy and condolences to the relatives of the 46 sailors killed in the March 26 strike.

The House "condemns North Korea in the strongest terms for sinking the ROKS Cheonan" and "calls for an apology by North Korea for its hostile acts and a commitment by North Korea never to violate the Korean War Armistice Agreement again," according to the resolution.

The measure urges the international community "to provide all necessary support to" South Korea as it "prepares to respond" to its northern neighbor's actions and calls for full implementation of UN sanctions against North Korea.

The resolution also says Washington "stands in solidarity" with South Korea and "reaffirms its enduring commitment to the alliance between the Republic of Korea and the United States and to the security of the Republic of Korea."

And it "further urges the United States, in coordination with its allies and partners, to take other appropriate actions in response to the sinking of the ROKS Cheonan and other hostile acts of North Korea," according to the text.

The House vote was the latest show of US Congress outrage at North Korea's alleged attack, which has sharply escalated regional tensions.

US Secretary of State Hillary Clinton has pressed China to get tough with the regime in Pyongyang even as she has warned against escalating the volatile standoff.