In his sternest message to Pyongyang since its Oct. 9 nuclear test, Bush vowed the United States would use whatever means necessary to keep North Korea from proliferating nuclear arms beyond its borders.
Bush, who has stressed the need to resolve the North Korea stand-off through diplomatic means, acknowledged it was still an open question whether UN Security Council sanctions would convince Pyongyang to abandon its nuclear program.
But he made clear that Washington, which has not ruled out military options, would take strong action if North Korea tried to sell nuclear weapons to America's enemies.
"They'd be held to account," Bush told ABC News when asked how he would handle North Korea if it attempted to send nuclear arms to Iran or al Qaeda.
"If we get intelligence that they're about to transfer a nuclear weapon, we would stop the transfer, and we would deal with the ships ... or the airplane that was dealing with taking the material to somebody," Bush told ABC News in an interview.
While refusing to specify how the United States would retaliate, he said, "You know, I'd just say it's a grave consequence."
Bush spoke as Rice pressed ahead with a diplomatic mission, assuring Japan that Washington would stand by its security commitments in the region as she sought to temper concerns of an Asian nuclear arms race.
Signalling possible behind-the-scenes efforts to reach a unified stand, Japan's Sankei newspaper cited diplomatic sources saying foreign ministers from the United States, Japan, South Korea, Russia and China would gather in Beijing on Friday to discuss North Korea's nuclear ambitions.
Chinese State Councillor Tang Jiaxuan was visiting Pyongyang to try to persuade North Korea to take part in the Beijing talks, the paper said.
"The United States has the will and the capability to meet the full range, and I underscore full range, of its deterrent and security commitments to Japan," Rice told a news conference in Tokyo, the first stop on a tour of North Asia.
The United States is worried Japan and South Korea might embark on an arms build-up in response to North Korea developing a nuclear arsenal.
"That is why it is extremely important to go out and reaffirm, and reaffirm strongly, US defence commitments to Japan and to South Korea," Rice told reporters travelling with her. Referring to an arms race, she said, "We have a lot of means to prevent that from happening."
Germany also warned that North Korea's underground test of a nuclear device on Oct. 9 and Iran's refusal to rein in its nuclear program could spark regional arms races.
RICE SEEKS UNIFIED STANCE
Rice is seeking a unified stance on UN sanctions slapped on Pyongyang last Saturday for exploding a nuclear device.
She made her trip as intelligence experts said satellites had spotted a pickup in activity at the North's suspected test site, suggesting a second blast may be imminent.
The US State Department on Wednesday cast doubt on media reports North Korea's military had informed China it intended to carry out a series of underground nuclear tests.
Assistant Secretary of State Christopher Hill acknowledged the possibility of further tests but said in an interview with National Public Radio's "Morning Edition," "We do not have any indication that it's going to happen imminently."
US officials said activity seen near suspected test sites was not conclusive.
Japan, a traditional target of Pyongyang's animosity, has seen debate increase over whether to acquire nuclear arms. Foreign Minister Taro Aso reiterated Tokyo had "absolutely no intentions now of preparing to possess nuclear weapons."
Some leading US Democratic senators, including Carl Levin of Michigan, joined a growing chorus of voices calling on the Bush administration to engage in bilateral talks with North Korea.

Copyright Reuters, 2006