The United States and Japan on Monday warned North Korea against a new nuclear test, with President Barack Obama vowing not to tolerate the communist state's "old pattern of provocation."

Obama welcomed Prime Minister Yoshihiko Noda for the first visit by a Japanese leader to the White House in three years, as the two leaders sought to show a personal chemistry to symbolize that the alliance was back on track.

Noda and Obama found common cause in pressing North Korea, which has threatened to retaliate and to reduce parts of South Korea to ashes due to the uproar over Pyongyang's defiant but failed rocket launch on April 13.

"What I've tried to do since I came into office (is) to make sure that North Koreans understand that the old pattern of provocation that then gets attention and somehow insists on the world purchasing good behavior from them, that that pattern is broken," Obama said.

"The more you engage in provocative acts, the more isolated you will become, the stronger sanctions will be in place," Obama said.

The Obama administration, after long hesitation, agreed on February 29 to deliver food aid to North Korea. It has suspended the pact after the rocket launch, which Washington believes was a disguised missile test.

While Obama declined to speculate on further actions by North Korea, Noda noted that the regime carried out its last nuclear test in 2009 amid an uproar over another such rocket launch.

"That means that there is a great possibility that they will conduct a nuclear test," Noda said.

Noda's visit comes after a rocky patch between the United States and Japan, with Obama aides aghast after the center-left Democratic Party of Japan swept into power in 2009 and flirted with moving closer to former nemesis China.

Noda is the party's third prime minister and the Obama administration has been upbeat about his leadership. Obama, as is his wont with close allies, joked before the cameras with Noda, noting that he has a black belt in judo.

Obama said that Noda had compared his own "leadership style to that of a point guard in basketball. He may not be the flashiest player, but he stays focused and gets the job done."

But polls show that Noda, like his predecessors, has suffered a precipitous fall in popularity less than a year after he took office. Noda has championed a controversial doubling of Japan's sales tax to close a ballooning debt.

Noda has also faced a backlash over his support -- announced before his first meeting with Obama in November -- for joining talks on the Trans-Pacific Partnership, an emerging US-led free trade pact across the Pacific Rim.

With Japanese farmers protesting that the trade deal would destroy their livelihoods, a joint statement between Obama and Noda said only that they "continue to advance our ongoing bilateral consultations" on the pact.

Days ahead of Noda's trip, the two countries tried to ease one key irritant in relations through an agreement to relocate 9,000 Marines from the Japanese island of Okinawa to the US territory of Guam.

Okinawa is home to half of the 47,000 US troops stationed in Japan under a post-World War II treaty, leading to local resentment. Japan and the United States maintained a 2006 plan to move an air station from a crowded city to a quiet seashore, despite calls by some activists to close the base completely.

Noda said that the two sides will "continue to work for an early resolution of this issue" and pledged to bolster defense ties.

Under last week's agreement, the two countries agreed to consider setting up the first joint training bases between the countries in Guam or the nearby Northern Mariana Islands, a significant step for officially pacifist Japan.

Noda later was toasted at a gala dinner by Secretary of State Hillary Clinton, who said that Americans were inspired by Japan's response to last year's tsunami devastation.

"Japan has remained an indispensable world leader even in the face of unthinkable tragedy," Clinton said.

S. Korea envoy to visit China amid nuke test concern
Seoul (AFP) May 1, 2012 - South Korea's chief nuclear envoy will travel to China this week, the foreign ministry said Tuesday, amid growing concern North Korea may test a nuclear bomb following its failed rocket launch last month.

Lim Sung-Nam will meet his counterpart Wu Dawei during the two-day visit starting Wednesday, said ministry spokeswoman Han Hye-Jin.

They would assess the situation following the launch on April 13 and discuss further action, she said.

China is the North's closest ally but supported a United Nations Security Council statement which strongly condemned the launch and tightened existing sanctions.

The North rejected the criticism of what it called an attempted peaceful satellite launch. The United States and its allies said the exercise was an excuse to test ballistic missile technology in defiance of a UN ban.

There is strong speculation the North will follow up with a nuclear test, as it did in 2006 and 2009 following UN censure of missile launches in those years.

Satellite photos show work underway at its underground test site.

Seoul is not aware of any signs of an imminent nuclear test, the spokeswoman said, but is closely watching the situation "with serious concerns".

In Washington Monday, US President Barack Obama warned North Korea that its "old pattern of provocation" was over and he would no longer follow a pattern of seeking to reward it for changing its ways.

Obama was speaking at a joint news conference with Japan's Prime Minister Yoshihiko Noda, who warned of a "great possibility" that the North would carry out a nuclear test.