Tensions on the Korean peninsula have risen since the North's tests
Chinese fishing boats are reported to be leaving the tense inter-Korean border in the Yellow Sea after North Korea's threat of military action. South Korea's Yonhap news agency quoted defence sources as saying the South's military authorities are trying to find out if Chinese ships were told to go.
The North conducted a nuclear test and fired several missiles this week.
On Friday it warned of "self-defence" measures if the UN Security Council imposed sanctions for the nuclear test.
"If the UN Security Council provokes us, our additional self-defence measures will be inevitable," the foreign ministry said in a statement carried by official media.
The communist nation has also threatened military action against the South after Seoul's decision to join a US-led Proliferation Security Initiative (PSI) under which North Korean ships could be stopped and searched. See comparison of North and South's military power
Pyongyang says this decision is tantamount to an act of war and that it is no longer bound by the Armistice which in 1953 brought an end to the Korean War. Fishing
"Chinese fishing boats operating near the Northern Limit Line (NLL) began withdrawing yesterday," Yonhap quoted a defence source as saying.
TROUBLESOME SEA BORDER
Known as Northern Limit Line
Position in Yellow Sea declared by UN in 1953
Not recognised by North
Deadly naval skirmishes along the line in 1999 and 2002
Regularly breached by North's fishermen
More than 280 Chinese vessels were fishing near the NLL for crab earlier this week but the number has fallen to about 140, according to the source.
The NLL, which North Korea refuses to recognise, marks the maritime border off the west coast of the peninsula. It was the scene of bloody naval clashes in 1999 and 2002.
Yonhap also quoted an official at Seoul's presidential office saying the withdrawal did not appear to be a sign of an imminent provocation.
"All Chinese ships would have evacuated the area if that was the case," the official said, adding the crab-catching season was almost over.
"Now that there's talk of... an all-out war, we fishermen are worried. Nowadays when we go out, we do know we are facing dangers," Reuters quoted a South Korean fisherman, Kim Jae-sik, in waters claimed by the North but occupied by the South.
US Defence Secretary Robert Gates, who is in Guam, told reporters he was not aware of any unusual troop movements in North Korea. Guam calm
Speaking at the Andersen US Air Force base, Mr Gates said he saw no need to increase troop levels in South Korea from the current total of 28,000.
"I am not aware of any military moves in the North, that are out of the ordinary at least," he said.
South Korea and the United States earlier raised the military alert level in the region which calls for increased surveillance but not increased military manoeuvres.
ESCALATING TENSIONS 27 May - North Korea says it is abandoning the truce that ended the Korean war and reportedly test-fires another missile 26 May - The North test-fires short-range missiles as South Korea announces it will join a US-led initiative to control trafficking in weapons of mass destruction 26 May - President Barack Obama pledges military support for America's East Asian allies, as the UN condemns the nuclear test 25 May - North Korea stages its second nuclear test, triggering international condemnation 29 April - Pyongyang threatens to carry out a nuclear test unless the UN apologises for criticising its recent rocket launch 14 April - Pyongyang says it is ending talks on its nuclear activities and will restore its disabled nuclear reactor after UN criticism of its rocket launch 5 April - The North goes ahead with a controversial rocket launch, seen by major governments as a cover for a long-range missile test
Analysts say the tension generated in the North could be largely geared to the domestic audience, to justify military spending and control of the population against an alleged threat from the West.
"We have no intention of taking military action against North Korea unless they do something that requires it," Mr Gates said, in comments described by AFP news agency as an attempt to play down the sense of crisis.
"Just based on the what the Chinese government has said publicly, they're clearly pretty unhappy about the nuclear testing in particular and they weren't very happy about the missile test either," he added.
Mr Gates said North Korea's actions had been "very provocative, aggressive, accompanied by a very aggressive rhetoric", and issued a warning against any military moves.
"I think at the same time it may create some opportunities for multilateral cooperation to try and persuade the North Koreans to change that behaviour," he said.
Mr Gates is on his way to a two-day security dialogue in Singapore.
In New York, the US and Japan circulated a draft United Nations Security Council resolution to key council members that condemned Pyongyang's second nuclear test on Monday and called for strict enforcement of UN sanctions imposed on North Korea after its first atomic test in October 2006.
When North Korea agreed in February 2007 to abandon its nuclear ambitions in return for aid and diplomatic concessions, there was real hope of reaching a settlement.
But the negotiations stalled as it accused its negotiating partners - the US, South Korea, Japan, China and Russia - of failing to meet agreed obligations.
Angharad is off to Seoul in June for five weeks and all this is about to kick off
Compared to Wales Seoul is a very civilized place
Joking aside, our company is heavily involved with South Korea. We have lots of people there and there isn't a "come-back-home-quick" warning for them so should be okay.
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