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 Seoul confirmed the launch of another short-range missile off North Korea's east coast.
The BBC's Chris Hogg says some reports claim the latest missile - the sixth - travelled further than the other five.
In another development on Friday, North Korea warned of "self-defence" measures if the UN Security Council imposed sanctions over 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.
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.
"Chinese fishing boats operating near the Northern Limit Line (NLL) began withdrawing yesterday," Yonhap quoted a defence source as saying.
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.
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.
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 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 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.