May 29: South Korean Navy sailors on speed boats aim their machine guns at Yeonpyong Island, west of mainland South Korea.
May 26: North Koreans are seen during a ceremony to celebrate the nation's underground nuclear test, in Pyongyang, North Korea.
An undated photo released in January 2009 shows a missile firing drill at an undisclosed location in North Korea.
May 24: A map of North Korea shows the epicenter of a 4.7 magnitude earthquake believed to have occurred as a result of the nuclear test.
File: The cooling tower of North Korea's Yongbyon nuclear complex, a plutonium-producing reactor, was demolished on June 27, 2008.
Apr. 7: In this image made from video, a rocket launch is seen in Musudan-ni, North Korea on April 5.
April 9: North Korean leader Kim Jong II attends the first session of Supreme People's Assembly of the country.
The launchpad in North Korea which houses the three-stage Taepodong-2 missile.
North Korea warned Friday it would take "self-defense" action if provoked by the United Nations Security Council, which is considering tough sanctions on the communist regime for conducting a nuclear test.
Tensions surrounding North Korea rose further as Chinese fishing boats pulled away from its coast, possibly to avoid skirmishes between the Koreas. But U.S. Secretary of Defense Robert Gates said the situation is not a crisis and no additional U.S. troops will be sent to the region.
"If the U.N. Security Council makes a further provocation, it will be inevitable for us to take further self-defense measures," the North's Foreign Ministry said in a statement carried by the official Korean Central News Agency.
The statement called the council "hypocrites."
"There is a limit to our patience," the statement said. "The nuclear test conducted in our nation this time is the Earth's 2,054th nuclear test. The five permanent members of the U.N. Security Council have conducted 99.99 percent of the total nuclear tests."
The North has been strident since its test — which it has also called a self-defensive measure. It did not specify what further action it was considering in response to U.N. resolutions, nor what it would consider a provocation.
Fears have increased of military skirmishes, particularly in disputed waters off the western coast, after North Korea conducted the nuclear test on Monday and then renounced the truce keeping peace between the Koreas since 1953.
The waters were the site of two deadly clashes in 1999 and 2002.
From Yeonpyeong, the South Korean island closest to the North, about a dozen Chinese ships could be seen pulling out of port in the North and heading elsewhere. South Korea's Yonhap news agency reported that more than 280 Chinese vessels were fishing in the area earlier this week, but the number has dropped to about 140.
It was not clear if the Chinese vessels, in the area for the crabbing season, were told by the North to leave or if they were leaving on their own for fear of clashes at sea.
"For now, it seems quiet," said local construction worker Lee Hae-un, 43. "But if North Korea provokes us with military power, I think our government should actively and firmly counteract it."
South Korean and U.S. troops facing North Korea raised their surveillance on Thursday to its highest level since 2006, when North Korea tested its first nuclear device. About 28,000 American troops are stationed across the South.
North Korea, whose 1.2-million strong military is one of the world's largest, says it is merely preparing to defend itself against what it says are plans by the United States to launch a pre-emptive strike to overthrow its communist government.
The United States has repeatedly denied any intention to attack North Korea.
In Washington, the Army's top officer, Gen. George Casey, expressed confidence that the U.S. could fight a conventional war against North Korea if necessary, despite continuing conflicts elsewhere.
But Gates, en route to Singapore for regional defense talks, tried to lower the temperature.
"I don't think that anybody in the (Obama) administration thinks there is a crisis," Gates told reporters aboard his military jet early Friday morning.
Meanwhile, talks at the United Nations Security Council over possible sanctions for the nuclear test were moving forward slowly.
Russia's U.N. ambassador said Thursday there was wide agreement among key world powers on what a new U.N. resolution should include, but said putting the elements together will take time because the issues are "complicated."
A list of proposals was sent Wednesday to the five permanent veto-wielding council members — the United States, Russia, China, Britain and France — and the two countries most closely affected by the nuclear test, Japan and South Korea.
Diplomats said a draft of the proposed resolution is not expected to be circulated until next week.
The two Koreas technically remain at war because they signed a truce, not a peace treaty, in 1953. North Korea disputes the U.N.-drawn maritime border off their west coast and has positioned artillery guns along the west coast on its side of the border, Yonhap said.
Traffic at the border between the Koreas appeared to be normal. Yonhap said more than 340 South Korean workers crossed to a joint industrial complex in the North.
The two Koreas are also maintaining a communication line to exchange information on commercial vessels passing through each other's waters, Unification Ministry spokeswoman Lee Jong-joo said.