WASHINGTON — Late in his first term, President Barack Obama announced a new emphasis on Asia and the Pacific Rim, a way of pivoting from the morass in the Middle East and focusing on a peaceful, booming region that the first Pacific-born president saw as the future.

He has visited Asia regularly since then — and will return at least twice again this year for economic and security summits.

But with notable exceptions, much of the administration's self-proclaimed “rebalance” has been dominated by emergency security matters, including North Korea's defiant nuclear tests, friction with China in the South China Sea, and the growing threat of cyber-espionage and digital theft.

On Thursday, with about 50 world leaders in Washington for the Nuclear Security Summit, only the leaders of China, Japan and South Korea received full, formal meetings with Obama, a sign of his priorities.

(The White House belatedly added a shorter session with France's President Francois Hollande, a critical ally in the war against Islamic State.)

“Trilateral security cooperation is essential to maintain peace and stability in Northeast Asia,” Obama told reporters as he met with South Korean President Park Geun-Hye and Japanese Prime Minister Shinzo Abe.

Later, in a separate meeting with Chinese President Xi Jinping, Obama stressed their commitment to “de-nuclearization of the Korean Peninsula” and praised China's new Nuclear Security Center of Excellence, which seeks to prevent smuggling of nuclear material.

“As the two biggest economies, China and the U.S. have a responsibility to work together,” Xi said.

The favored treatment for the three Asian powers underscores both the importance the White House attaches to boosting trade and security in Asia as well as the urgency of dealing with North Korea's nuclear saber-rattling and other potential conflicts.

“The rebalance (toward Asia) … the reshuffling of U.S. priorities … is one of the most significant strategic initiatives of Obama's tenure,” said Elizabeth Economy, director of Asia studies at the nonpartisan Council on Foreign Relations.

The meetings come against the backdrop of a tumultuous U.S. presidential campaign that potentially could upend a half-century of U.S. policy in northeast Asia.

Republican front-runner Donald Trump has said in recent days that South Korea and Japan may need to develop their own nuclear weapons to confront regional threats, rather than relying on the U.S. nuclear umbrella that has shielded them for decades.

“You have so many countries right now that have them,” he said Tuesday in a CNN town hall. “Wouldn't you rather ... have Japan have nuclear weapons when North Korea has nuclear weapons?”

Whether a whim or a serious notion by Trump, the issue came up in multiple press briefings ahead of the nuclear security summit, the fourth in a series launched by Obama in 2009 as part of his stated goal of seeing a world without nuclear weapons.

Ben Rhodes, the White House deputy national security adviser, said a pillar of U.S. foreign policy has been to prevent the spread of nuclear weapons, not to encourage it, as Trump suggests. “That's been the position of bipartisan administrations, everybody who's occupied the Oval Office,” Rhodes said. “Frankly, it would be catastrophic were the United States to shift its position.”

White House aides say Obama's carefully nurtured relationship with Xi — their meeting Thursday was their eighth face-to-face session — was critical in getting Chinese support at the U.N. Security Council on March 2 to impose stiff sanctions on North Korea, Beijing's ally, in response to its latest nuclear test.

China has expressed its own frustration with North Korea's ruler, Kim Jong Un.

Beijing was caught off guard by Pyongyang's most recent underground nuclear test, on Jan. 6, the eve of the Chinese New Year. Beijing had told Kim's government not to conduct the test, its first 2013.

Tensions with China also have been growing in the resource-rich and bitterly-contested South China Sea.

Also Thursday, the White House announced that Washington and Beijing will sign on April 22 the so-called Paris Agreement, which contains pledges to reduce environment-damaging emissions. The U.S. and China are the world's biggest carbon emitters.

Tribune Newspapers' W.J. Hennigan contributed.

tracy.wilkinson@tribpub.com