A defiant North Korea threatened more nuclear tests Wednesday and said impending sanctions would be considered an act of war as nervous neighbors raced to bolster defenses and impose punitive measures on Pyongyang.

In its first formal statement since Monday's claimed atomic bomb test, Pyongyang hailed the blast as a success and warned that any act to penalize North Korea would be met with physical retaliation.

"If the U.S. keeps pestering us and increases pressure, we will regard it as a declaration of war and will take a series of physical corresponding measures," the North's Foreign Ministry warned in a statement carried by the official Korean Central News Agency. The statement did not specify what those measures would be.

South Korea said it was making sure its troops were prepared for nuclear warfare, and Japan announced a ban on all imports from the communist nation.

In Washington, President Bush called for stiff sanctions on North Korea for its reported nuclear test, and asserted the United States has "no intentions of attacking" the reclusive regime.

In a Rose Garden news conference, Bush said the United States remains committed to diplomacy, but also "reserves all options to defend our friends in the region."

Meanwhile at the United Nations, Secretary-General Kofi Annan urged the United States to hold bilateral talks with North Korea and called on the communist nation not to escalate the situation.

"I would urge the North Korean authorities not to escalate the situation any further," Annan told reporters. "We already have an extremely difficult situation."

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The United States said it hopes to circulate a revised U.N. resolution on North Korea soon, with the U.S. still at odds with China over how strong sanctions against North Korea should be.

The five permanent council members -- the U.S., China, Russia, Britain and France -- met Wednesday morning with Japan, which hold the Security Council presidency this month, and U.S. Ambassador John Bolton said afterwards: "We'll try to circulate a revised text later this afternoon."

"There are a number of disagreements," Bolton said. "We think the fact that North Korea has conducted a nuclear test does amount to a clear threat to international peace and security and warrants action under Chapter 7 of the U.N. Charter as well as a variety of strong measures."

"There's not agreement on all of those points, so we're continuing to press ahead and we'll have to see what further discussions entail," he said.

Chapter 7 includes a range of measures to deal with threats to international peace and conflicts, ranging from breaking diplomatic relations and imposing naval blockades to military action.

North Korea's No. 2 leader Kim Yong Nam threatened in an interview with a Japanese news agency that there would also be more nuclear tests if Washington continued what he called its "hostile attitude."

Kim, second to North Korean leader Kim Jong Il, told Kyodo News agency that further nuclear testing would hinge on U.S. policy toward his communist government.

"The issue of future nuclear tests is linked to U.S. policy toward our country," Kim Yong Nam was quoted as saying when asked whether Pyongyang will conduct more tests.

Along the razor-wired no-man's-land separating the divided Koreas, communist troops were more boldly trying to provoke their southern counterparts: spitting across the demarcation line, making throat-slashing hand gestures, flashing their middle finger and trying to talk to the troops, said U.S. Army Maj. Jose DeVarona of Fayetteville, N.C., adding that the overall situation was calm.

North Korea's 1 million-member military is the world's fifth-largest.

On the streets of North Korea's capital, it seemed like business as usual. Video by AP Television News showed people milling about Kim II Sung square in Pyongyang and rehearsing a performance for the 80th anniversary of the "Down with Imperialism Union."

South Korean Defense Minister Yoon Kwang-ung said that Seoul could enlarge its conventional arsenal to deal with a potentially nuclear-armed North Korea.

Scientists and other governments have said Monday's underground test has yet to be confirmed, with some experts saying the blast was significantly smaller than even the first nuclear bombs dropped on Japan during World War II.

North Korea appeared to respond to that Wednesday, saying in its statement that it "successfully conducted an underground nuclear test under secure conditions."

In rare direct criticism of the communist regime from Seoul, South Korean President Roh Moo-hyun said the security threat cited by North Korea "either does not exist in reality, or is very exaggerated," according to South Korea's Yonhap news agency.

He spoke even as South Korea's military was checking its readiness for nuclear attack, Yonhap said. The Joint Chiefs of Staff recommended improving defenses, possibly with state-of-the-art weapons to destroy nuclear missiles, the report said.

The top U.S. general in South Korea said American forces are fully capable of deterring an attack despite the North's still-unconfirmed nuclear test.

"Be assured that the alliance has the forces necessary to deter aggression, and should deterrence fail, decisively defeat any North Korean attack against" South Korea, U.S. Army Gen. B.B. Bell said in a statement to troops. "U.S. forces have been well- trained to confront nuclear, biological and chemical threats."

About 29,500 U.S. soldiers are deployed in the South, a remnant of the 1950-53 Korean War that ended in a cease-fire, not a formal peace treaty.

Bell said seismic waves detected after the claimed test were still being analyzed and it had not been yet determined if the test was successful.

Japan took steps to punish North Korea for the test, prohibiting its ships from entering Japanese ports and imposing a total ban on imports from the impoverished nation.

North Korean nationals are also prohibited from entering Japan, with limited exceptions, the Cabinet Office said in a statement released after an emergency security meeting late Wednesday.

"We cannot tolerate North Korea's actions if we are to protect Japanese lives and property," Prime Minister Shinzo Abe said after an emergency security meeting late Wednesday. "These measures were taken to protect the peace."

A total ban on imports and ships could be disastrous for North Korea, whose produce such as clams and mushroom earns precious foreign currency on the Japanese market. Ferries also serve as a major conduit of communication between the two countries, which have no diplomatic relations.

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Two dozen North Korea-registered trade ships are moored at Japanese ports, according to public broadcaster NHK. Local traders already were refusing to unload shipments to protest the alleged test, and the boats were expected to be ordered out, NHK said.

Tokyo already has halted food aid and imposed limited financial sanctions against North Korea after it test-fired seven missiles into waters between Japan and the Korean peninsula in July, including one capable of reaching the United States.

The North lashed out at the prospect of further economic sanctions.

"The enemy schemes to destroy us through economic lockout ... but that is merely a foolish illusion," said an editorial published by the state-run Rodong Sinmun, according to Radio Press.

Japanese TV reports that North Korea may have conducted a second nuclear test stirred new anxieties, but one of the networks later issued a retraction and officials said it was most likely a false alarm.

NHK and Nippon Television, a commercial network, reported that "tremors" had been detected in North Korea, leading the government to begin investigating whether a second blast had taken place. The reports cited unidentified government sources. Nippon Television later apologized.

South Korean and U.S. seismic monitoring stations said they hadn't detected any indications of a second test, findings backed by White House spokesman Blair Jones.

The Associated Press contributed to this report.