WASHINGTON » Japan’s new prime minister declared today he would make his country a stronger U.S. ally and joined President Barack Obama in warning North Korea that its recent nuclear provocations would not be tolerated.

After meeting Obama in the Oval Office, Prime Minister Shinzo Abe also sent a clear message to China: that while Japan does not want confrontation with Beijing, it won’t tolerate challenges to its sovereignty over islands disputed by the two Asian powers.

Those regional tensions served as the backdrop for today’s meetings that came just two months after Abe began his second stint as Japan’s prime minister following a convincing election victory.

Obama said he and Abe were united in their "determination to take strong actions" in response to North Korea’s nuclear test this month, which followed a successful long-range rocket launch last month. That has propelled the isolated, authoritarian state closer to having a weapon of mass destruction that could threaten the U.S.

Abe said he and Obama have agreed to push for tougher sanctions by the U.N. Security Council and spelled out why Pyongyang’s actions are cause for worry.

"They (North Korea) have increased the range of their missile immensely and have attained the ability to reach even the mainland United States," Abe said at a Washington think tank after his White House visit. He said Pyongyang was also claiming it has made a smaller nuclear bomb that could be delivered by missile.

Speaking through a translator, the Japanese leader said this was why the United States was pressuring China to exert more influence over its North Korean ally. Abe said it was important for the entire international community to do the same.

Most experts believe North Korea is still some years away from being able to hit America, although its shorter-range missiles could already threaten its neighbors.

Abe, a nationalist and advocate of U.S. relations with the United States, is the latest in a revolving door of Japanese prime ministers — the fifth since Obama took office. That’s made it difficult to establish a personal rapport between Japanese and U.S. leaders, notwithstanding the enduring nature of the bilateral relationship. Japan hosts about 50,000 American forces and is a cornerstone of Washington’s Asia policy

Abe outlined his policy to revive his nation after years of malaise by building a strong economy and strong national defense.

"Japan is not, and will never be, a tier-two country. That is the core message I am here to make. And I reiterate this by saying, I am back, and so shall Japan be," Abe said.

He promised to enhance Japan’s role in international affairs, build its cooperation with other democracies and promote open use of the seas and rules-based trade.

Japan’s relationship with Washington has assumed more importance for Tokyo in recent months as it has locked horns with China over the control of unoccupied islands in the resource-rich seas between them.

The dispute flared after Tokyo nationalized some of the islands in September. China also claims the tiny islands, which it calls Diaoyu. It has stepped up patrols into what Japan considers its territorial waters, heightening concern that a conflict could be sparked. The tensions highlight the rivalry between China, the world’s second-largest economy, and Japan, which is the third.

Obama did not address the dispute in his brief remarks, but separately, Secretary of State John Kerry complimented Japan on the restraint it has shown and its efforts to prevent a "significant confrontation."

Abe said Japan had no intention to escalate the dispute and that his door was always open to Chinese leaders.

But he had some words of defiance, too, over Japan’s sovereignty of the islands.

"We simply cannot tolerate any challenge now and in the future," Abe said. "No nation should make any miscalculation about firmness of our resolve. No one should ever doubt the robustness of the Japan-U.S. alliance."

The U.S. has treaty obligations to help Japan in the event of a conflict, obligations Abe said were a stabilizing factor in ensuring peace and stability in the region.

In comments that will be welcomed by Washington, Abe held out an olive branch to South Korea, a key U.S. ally that shares Japan’s concern over North Korea’s provocations.

He said the Japan-South Korea relationship was "extremely important" and he wanted to resolve the differences between them. The two Asian democracies have bickered over another island dispute, and Seoul believes Tokyo lacks contrition for its colonial past and use of Korean sex slaves during World War II.

Today’s meeting was an opportunity for the U.S. to gauge Tokyo’s intent to join negotiations on the Trans-Pacific Partnership, a regionwide free-trade pact pushed by Washington. Abe held back from such a commitment, which is opposed by most of his party and Japan’s small but politically powerful farming lobby, at least until after key elections in July for the upper house of the legislature, known as the Diet.

In a joint statement following the meeting, the two leaders agreed to continue their talks about Japan’s "possible interest" in joining the trade pact, known as the TPP. But they agreed that concerns remained, particularly with respect to the automotive and insurance sectors.