SEOUL, South Korea – South Korea’s president embarked yesterday on a one-day whirlwind trip to Washington to forge a united front with President Bush on dealing with North Korea, after Pyongyang dealt another setback to reconciliation with new nuclear boasts.

Based on the North’s bold claims, Japan said yesterday it believed the communist nation’s nuclear weapons programs were “considerably advanced.” However, the Japanese Defense Agency’s administrative deputy director, Takemasa Moriya, acknowledged that Tokyo’s assessment relied on Pyongyang’s actions and announcements, not hard evidence.

American officials expressed optimism earlier this week after meetings with North Korean diplomats in New York that long-stalled nuclear disarmament talks would resume. But the North did not give any date for its return to the six-nation negotiations – which also include China, Japan, Russia, and South Korea – and has reverted to its usual bombast in recent days.

North Korean Vice Foreign Minister Kim Gye Gwan told ABC News in Pyongyang that the communist nation had “enough nuclear bombs to defend against a U.S. attack” without being more specific, and he said it was building more. Mr. Kim also hinted his country’s weapons scientists could mount nuclear warheads on missiles.

White House press secretary Scott McClellan said yesterday that Pyongyang’s claims to have built more bombs “only further isolate North Korea from the international community.”

The North has never tested a nuclear weapon, but it is widely believed to have enough plutonium for about a half-dozen bombs and recently made moves allowing it to harvest more nuclear material from its main reactor.

Pyongyang has stayed away from arms talks for nearly a year, citing “hostile” American policies. But President Roh of South Korea is keeping to his policy of engagement with the North despite its recalcitrance – deepening divisions with Seoul’s main ally Washington, who views Pyongyang as a rogue regime at risk of proliferating weapons of mass destruction.

Mr. Roh’s meeting and lunch today with President Bush is intended for the sides to patch up differences and reaffirm their alliance.

“There are very serious strains in the [U.S.-South Korean] relationship,” Seoul-based director of the North East Asia Project for the International Crisis Group, Peter Beck, said. “The threat of divorce is real – both sides are increasingly asking questions that weren’t being asked a few years ago.”

This year, Secretary of State Rice called the North one of the world’s “outposts of tyranny” after Mr. Bush laid out a sweeping vision of spreading democracy throughout the world.

Mr. Bush reiterated in a Wednesday interview with Fox News that Washington had not ruled out the possibility of referring Pyongyang to the United Nations for sanctions.

“Well, that is an option down the road,” he said.

Mr. Roh has come out against regime change in the North and even expressed understanding of why Pyongyang wants nuclear weapons, purportedly for self-defense.

Mr. Roh also has expressed concern about reforms in the American military to create a more flexible force – raising worries American troops here could become embroiled in regional conflicts, particularly between Taiwan and China.

The potential of restrictions on American troop operations reportedly led an American defense official to threaten their withdrawal.

South Korea’s Foreign Ministry acknowledged yesterday an unannounced visit by U.S. Deputy Undersecretary of Defense Richard Lawless this week but refused to disclose what was discussed. Local newspapers reported Mr. Lawless said Washington might have to withdraw its troops if Seoul keeps disagreeing on a range of issues, including Pentagon plans for American forces to potentially operate across the region.

The South has recently refused American demands to shoulder more of the cost for American troops’ presence, with the sides signing an agreement yesterday for Seoul to pay $681 million this year and next, down 9% from last year.

Ahead of his trip, Mr. Roh met Wednesday in Seoul with top American military commanders to underscore cooperation with Washington.

“Since I became president, there have been many changes in the alliance between Korea and the United States; whether they are good or bad are subject to interpretation,” Mr. Roh said.

Still, he stressed Seoul’s alliance with America was the basis for its success as a democracy and market economy.

“This fact remains true today and will not change in the future,” Mr. Roh said.