By Jeff Mason and Matt Spetalnick

WASHINGTON (Reuters) - U.S. President Barack Obama joined with South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe on Thursday to present a united front over what he called "provocations" committed by North Korea in its recent nuclear and missile tests.

Meeting on the sidelines of a global nuclear security summit in Washington, the three leaders recommitted their countries to each other’s defense and warned they could take further steps to counter threats from Pyongyang.

Obama held separate talks with President Xi Jinping of China, the closest North Korea has to an ally, and said they both wanted to see "full implementation" of the latest United Nations sanctions against Pyongyang. But Xi offered no sign that Beijing was prepared to go beyond its consent to the Security Council measures imposed in early March.

“We are united in our efforts to deter and defend against North Korean provocations,” Obama told reporters after the U.S.-Japan-South Korea meeting. "We have to work together to meet this challenge."

Relations between Park and Abe have been frosty in the past, but the two have been brought together in recent months by shared concerns about North Korea, which conducted a fourth nuclear bomb test on Jan. 6 and launched a long-range rocket into space in February.

The United States has sought to encourage improved ties between Seoul and Japan, its two biggest allies in Asia, given worries not only about North Korea but also an increasingly assertive China.

The expanded U.N. sanctions aimed at starving North Korea of funds for its nuclear weapons and ballistic missile programs were approved in a unanimous Security Council vote on a resolution drafted by the United States and China.

But even though Beijing has signed on, doubts persist in the West on how far it will go in tightening the screws on impoverished North Korea, given Chinese concerns about fueling instability on its borders.

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Appearing later with Obama, Xi said that while Washington and Beijing disagree in some areas, they have had "effective communication and coordination" on the North Korean issue.

However, China, considered the most capable of influencing North Korea’s reclusive leadership, has said repeatedly that sanctions are not the solution and only a resumption of international talks can resolve the dispute with Pyongyang. Six-party talks between the two Koreas, China, the United States, Japan and Russia aimed at curbing the North's nuclear ambitions collapsed after the last round in 2008.

Thursday's meetings took place as leaders from more than 50 countries gathered in Washington for a two-day summit hosted by Obama and focused on securing vulnerable atomic materials to prevent nuclear terrorism. North Korea’s nuclear defiance was also high on the agenda.

Notably absent is Russian President Vladimir Putin, adding to doubts that a meeting without one of the world’s top nuclear powers present can yield major results.

Despite that, a joint U.S.-China statement showed the two countries, while rivals on trade and at odds over the South China Sea, agreeing to work together to investigate and curb nuclear smuggling and to hold annual bilateral talks on the issue.

'MISGUIDED CALCULUS'

Obama said he, Park and Abe had directed their teams to come up with additional steps they can take collectively against North Korea.

Park said the leaders had discussed ways to force North Korea to "alter its misguided calculus" on its weapons programs, and Abe expressed a commitment to strengthening trilateral security cooperation.

The meeting came just days after Republican presidential frontrunner Donald Trump caused an uproar by suggesting that Japan and South Korea should be allowed to build their own nuclear arsenals, putting him at odds with decades of U.S. policy.

Ben Rhodes, Obama’s deputy national security adviser, said Trump’s comments did not come up in the three leaders’ discussions. But he said: “It would be catastrophic were the United States to shift its position and indicate that we support somehow the proliferation of nuclear weapons to additional countries.”

Obama has less than 10 months left in office to follow through on one of his signature foreign policy initiatives – locking down as much of the world’s dangerous nuclear materials as possible - and this week's meeting is his fourth and final Nuclear Security Summit.

While progress has been made, some arms-control advocates say the diplomatic process seems to have lost momentum and could slow even further once Obama leaves office in January.

A boycott by Russia, apparently unwilling to join in a U.S.-dominated gathering at a time of increased tensions between Washington and Moscow especially over the Ukraine conflict, could detract from any decisions made at the summit.

The deadly militant bomb attacks in Brussels this month have fueled concern that Islamic State could eventually target nuclear plants, steal material and develop radioactive “dirty bombs,” a topic that may well be uppermost in leaders’ minds as they meet.

(Additional reporting by Susan Heavey, Roberta Rampton, David Brunnstrom and Doina Chiacu; Writing by Matt Spetalnick; Editing by Frances Kerry and James Dalgleish)