BEIJING — North Korea was suspected yesterday of preparing to stage a second nuclear test.

American satellite pictures and officials in Japan and South Korea suggested that “suspicious vehicle movements” had been observed around the first test site, near North Korea’s northeastern city of Kilju.

Japan’s foreign minister, Taro Aso, said North Korea might be preparing a follow-up to the test last week, which proclaimed its nuclear capability and provoked international condemnation and U.N. sanctions.

In perhaps the strongest comments yet on the crisis by an American official, the assistant secretary of state, Christopher Hill, warned the regime in Pyongyang that it risked paying a “very high price” if it continued to saber-rattle over its atomic-weapons program.

A second test would be “a very belligerent answer” to the international community, which “would have no choice but to respond very clearly,” Mr. Hill said in Seoul, the South Korean capital.

“We have to make it clear that North Korea will have to pay a very high price for this reckless behavior,” he said.”We need to work very hard with our partners and allies to implement the U.N. Security Council resolution.”

A South Korea official confirmed the activity at the test site. However, he said no intelligence indicated that a test was imminent.

Technical assessments of the first blast, which proved to be of very low strength for a nuclear test, had given rise to speculation that the North Koreans might need a second one.American seismologists and nuclear scientists, working from evidence of the shock waves, reckoned it to have a strength about half a kiloton, compared with more than 12 kilotons for the bomb dropped by America on Hiroshima in 1945.

An assessment by the Federation of American Scientists suggested that the first test had been a partial failure, meaning a second attempt was likely.

Other specialists said North Korea could have been deliberately testing a “micro-bomb” that it was capable of installing on a missile, or that it deliberately produced a “dummy run” for a more significant test later.

North Korea itself made no mention of a second test in its first full reaction yesterday to the vote in the U.N. Security Council during the weekend imposing sanctions.

Instead, it fulminated that the vote was “immoral behavior devoid of impartiality,” using significantly harsher vocabulary even than is usual for the North Korean regime.

“The UNSC ‘resolution’, needless to say, cannot be construed otherwise than a declaration of a war against the DPRK because it was based on the scenario of the U.S., keen to destroy the socialist system,” it said. Both the statement and the second test will add to the atmosphere of tension overhanging a visit to Japan, South Korea, and China by Secretary of State Rice starting today.

Both South Korea and China fear an incident — perhaps beginning with attempts to intercept a North Korean cargo ship under the sanctions regime — will spiral out of control into conflict. The North’s statement said it would “deal merciless blows” against anyone who violated its sovereignty.

Part of Ms. Rice’s mission will be to hold both South Korea and China to what originally seemed more assertive positions toward the North. She will almost certainly be trying to discover what China’s ambassador to the United Nations, Wang Guangya, meant by saying after voting for the sanctions package that China “did not approve” of the inspection program.

He has since said China will carry out “inspections” but not “interceptions” or “interdictions,” which suggests that it will carry out sporadic customs checks at the border but not attempt to stop vessels at sea.

But because of China’s extensive land border with North Korea, America is relying on China to implement the resolution, which the Bush administration believes implies an acceptance that Kim Jong Il’s regime must be squeezed.

Prime Minister Han of South Korea was also cautious. “Sanctions against North Korea should be done in a way that draws North Korea to the dialogue table,” she said.