North Korea's nuclear test and missile launches caused alarm in the South
There are signs that North Korea may be planning more missile launches, US defence officials say.
Defence officials in Washington said US satellite photos had revealed vehicle activity at a site in North Korea used to fire long-range missiles.
The vehicle movements resembled activity before North Korea fired a long-range rocket last month, the officials said.
The North fired another short-range missile on Friday, the sixth this week.
The officials, speaking on conditions of anonymity, said the US was closely monitoring the North's missile sites and other sensitive facilities.
However, a Pentagon official told the BBC that the US had noticed this type of activity on previous occasions which did not always lead to a missile being fired.
The official said activity at this site had been going on for more than a couple of days, but did not know for how long exactly.
The North conducted a nuclear test as well as firing missiles this week.
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
On Friday North Korea warned of "self-defence" measures if the UN Security Council imposed sanctions over its 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.
As global concern over Pyongyang's actions mounted, Chinese fishing boats were reported to be leaving the tense inter-Korean border in the Yellow Sea amid fears 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.
"Chinese fishing boats operating near the Northern Limit Line (NLL) began withdrawing yesterday," the source said.
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.
The hardline communist state, under President Kim Jong-il, 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.
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.
In April, North Korea's launch of a long-range rocket was portrayed by Pyongyang as a peaceful move connected to communications satellite technology.
However, the US and its allies said the launch was meant to be a test of a long-range ballistic missile, and failed to send a probe into space.
Meanwhile, initial US government tests to determine if North Korea did detonate a nuclear device on Monday are so far "inconclusive," a US official said on Friday.
Tests for radioactivity in air samples taken from the region were still under way but initial analysis did not confirm Pyongyang fired an atomic bomb, the official told AFP news agency.
"The results are not in yet," another official said.
Experts say seismology readings taken at the time when North Korea claims the test took place are consistent with an atomic explosion.
The last time North Korea detonated a nuclear device was in 2006, and US officials then said radioactive debris in air samples confirmed a nuclear device had been detonated underground.