At her side, Rice's Japanese counterpart drew a firm line against his nation developing a nuclear bomb.
The top U.S. diplomat said she reaffirmed President Bush's pledge, made hours after North Korea's Oct. 9 underground test blast, "that the United States has the will and the capability to meet the full range — and I underscore the full range — of its deterrent and security commitments to Japan."
Rice spoke following discussions with Japanese Foreign Minister Taro Aso, the first stop on her crisis mission to respond to the threat posed by the North.
Back home, President Bush told ABC News that if the U.S. learned North Korea was about to transfer nuclear technology to others, the communist nation would face "a grave consequence." He did not elaborate.
"I want the leader to understand — the leader of North Korea to understand that he'll be held to account," Bush said, referring to the country's ruler, Kim Jong Il.
There were continued signs Wednesday that North Korea might be readying for a second nuclear test, perhaps while Rice was in Asia this week.
China's president, Hu Jintao, apparently has sent a special envoy to North Korea, according to a former South Korean lawmaker, Jang Sung-min, citing diplomatic officials in Beijing. Rice planned to see the Chinese official, State Councilor Tang Jiaxuan, this week in Beijing.
There were reports North Korea had told China it was ready to conduct up to three more nuclear tests. At the State Department in Washington, spokesman Tom Casey said, "We certainly haven't received any information from them, from the Chinese, that they've been told by Pyongyang that another test is imminent."
U.S. government officials, who spoke on condition of anonymity because of the sensitive situation, said there was no evidence to suggest a second test was imminent.
But given the underground nature of the testing, officials said, it could happen with little or no warning.
In Seoul, South Korea, the country's foreign minister — the incoming U.N. secretary-general — warned the North not to detonate a second nuclear test.
"If North Korea conducts an additional test, the response of the international community will be much more serious," Ban Ki-moon said.
In her meeting Thursday morning with Japanese Prime Minister Shinzo Abe, Rice reaffirmed Washington's defense commitment and "underscored the strength of our alliance," her spokesman said.
The officials discussed ways of cooperating on carrying out terms of the U.N. resolution that penalizes North Korea for its test. Steps include boarding and inspecting the North's ship for banned weapons, though China has expressed reluctance to do so.
"Insuring the implementation of the U.N. Security Council resolution will lead to changing the North Korean policies," Abe said. "In order to do so, we will work out the details of what we can do and what we need to do on the working level."
Rice's reference to U.S. willingness to honor the "full range" of the nation's security commitments was meant to show that the United States does not want to see its allies on a nuclear arms race to protect themselves. It also was likely to be taken as a reminder to North Korea that, should it use nuclear weapons on a neighbor, the U.S. has powerful forces of its own and is pledged to defend its friends.
The U.S. is concerned that Japan, South Korea and perhaps Taiwan may want to develop their own nuclear weapons programs to counter North Korea. Such moves would anger China, which has nuclear weapons, and raise tensions in Asia.
While North Korea is seeking direct negotiations with the U.S., the Bush administration is committed to six-nation disarmament talks, which have stalled. Rice's spokesman, Sean McCormack, said before the secretary of state left for Seoul that there are no plans for the U.S., China, Japan, Russia and South Korea to hold a strategy session this week in Beijing.
North Korea contends it needs nuclear weapons to counter U.S. aggression. The U.S. has said it does not intend to attack the North or topple its communist government.
North Korea has a standing army of about 1.2 million, with millions more in reserve, and a supply of missiles capable of reaching Asian cities. North and South Korea technically are still at war more than 50 years after the Korean conflict ended.
The U.S. has 29,500 troops in South Korea, plus other air and naval forces in range. While the U.S. has no land-based nuclear weapons in Asia, it does have submarines equipped with nuclear weapons.
Japan, home to more than 35,000 U.S. troops, was Rice's first stop on a four-day tour of Asia and Russia.
"The United States has no desire to escalate this crisis. We would like to see it de-escalate," Rice told reporters.
North Korean leader Kim Jong Il made his first known public appearance since his country's recent nuclear test, attending a performance of songs praising him, the North's official media reported Wednesday. There was no mention of the nuclear test in the report.
The nuclear explosion has drawn strong international condemnation and U.N. penalties that the North has rejected. The North, in turn, has threatened further unspecified moves.
Even discussing the issue is sensitive in Japan, with its troubled military history and its experience as the only nation where nuclear weapons were used in wartime.
"The government is absolutely not considering a need to be armed by nuclear weapons," Aso said with Rice at his side. "We do not need to acquire nuclear arms with an assurance by Secretary of State Rice that the bilateral alliance would work without fault."
Later Wednesday, Abe insisted his government would not even discuss building a nuclear bomb.
"That debate is finished," Abe testily told reporters.