SEOUL, South Korea (AP) – North Korea stoked regional tensions Wednesday, threatening more nuclear tests and saying additional sanctions imposed on it would be considered an act of war, as nervous neighbors raced to bolster defenses and punish Pyongyang.

South Korea said it was making sure its troops were prepared for atomic warfare, and Japan imposed new economic sanctions to hit the economic lifeline of the communist nation’s 1 million-member military, the world’s fifth-largest.

North Korea, in its first formal statement since Monday’s claimed atomic bomb test, hailed the blast as a success and said attempts by the outside world to penalize North Korea with sanctions would be considered an act of war.

Further pressure will be countered with physical retaliation, the North’s Foreign Ministry warned in a statement carried by the official Korean Central News Agency.

“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 statement, said without specifying what those measures could be.

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

Mr. Kim, second to North Korean leader Kim Jong Il, told Kyodo News agency that further nuclear testing would hinge on American 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.

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,President Roh 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 American 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 American 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.

General 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 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.

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.

Meanwhile, 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 American seismic monitoring stations said they hadn’t detected any indications of a second test, findings backed by White House spokesman Blair Jones.

With the United Nations debating how to respond to North Korea, China agreed to punishment but not the severe sanctions backed by America.

Beijing is seen as having the greatest outside leverage on North Korea as a traditional ally and top provider of badly needed economic and energy aid.

America asked the U.N. Security Council to impose a partial trade embargo including strict limits on Korea’s weapons exports and freezing of related financial assets.

All imports would be inspected too, to filter materials that could be made into nuclear, chemical or biological weapons.