North Korea's march toward having a nuclear-tipped missile that could reach the US mainland is among the pressing national security priorities President Donald Trump faces. He has vowed it "won't happen" but has yet to articulate a strategy to stop it.

A wide array of options are on the table, but aggressive behavior by Pyongyang in response to US-South Korean military drills that began last week could further shrink chances for diplomatic engagement.

Upheaval in the administration has added to uncertainty in foreign capitals about how Trump's "America First" mantra will translate into foreign policy, and how a new president with no prior experience in government might handle a security crisis.

An administration official told The Associated Press Monday that tougher sanctions, military action and resumption of long-stalled negotiations with North Korea are all under consideration as part of a policy review to provide options for the president within weeks.

The official, who demanded anonymity to discuss the private deliberations, did not anticipate an immediate US response to the North's test-firing of four banned ballistic missiles Monday that South Korean and Japanese officials said flew about 1,000 kilometers (620 miles). Three of the missiles landed in waters that Japan, a close US ally, claims as its exclusive economic zone.

North Korea typically reacts during the annual military drills that it considers an invasion rehearsal, although Washington and Seoul say they are routine.

This year's response could be more heated than usual. Victor Cha, a former White House adviser on Asia, said North Korea tends to up the tempo of missile tests during the drills when relations with the US are bad. And next week, the drills shift from table-top exercises to military maneuvers.

"I think there are more tests coming," Cha said.

The US and Japan have requested an emergency meeting of the UN Security Council to discuss the latest missile launches. The meeting is likely to take place Wednesday, a UN diplomat said, demanding anonymity to speak before the official announcement.

North Korea, meanwhile, urged the council to discuss the US-South Korea exercises, asserting the drills are driving the region toward "nuclear disaster."

Ri Song Chol, counsellor at North Korea's UN mission, told AP that supreme leader Kim Jong Un has said as long as there are "military exercises in front of the gate of my country," the North will continue to strengthen its military forces and "pre-emptive attack capabilities."

Over the seven weeks of last year's exercises, North Korea conducted nine missile tests, including of submarine-based and intermediate range missiles, but never more than two missiles at once. Five of the tests failed.

Cha said that Trump's hand could be forced by North Korea's provocative actions. The Obama administration relied heavily on sanctions, but the moves failed to stop Pyongyang.

"Right now they don't have any choice. I mean they've already had two sets of missile tests and then the use of a chemical weapon in an airport," Cha said.

North Korea is the prime suspect in the assassination last month of Kim Jong Un's estranged half brother in Malaysia, using what authorities say was VX nerve agent.

David Wright at the Union of Concern Scientists said the missiles launched Monday were likely either extended-range Scuds or medium-range Nodong ballistic missiles that have been tested numerous times before — not an intercontinental missile that threatens America.

"But the tests naturally will increase political pressure on Trump to take a tough stand," said Mark Fitzpatrick at the International Institute for Strategic Studies think tank. "He has a political imperative to show attention to the North Korean security threat, so as to counter the impression of a White House in disarray."

Trump's new national security adviser, H.R. McMaster, spoke by phone Monday with his South Korean counterpart Kim Kwan-jin, and they agreed to boost cooperation to get the North to face more effective sanctions and pressure, according to South Korea's presidential office.

"The United States stands with our allies in the face of this very serious threat," White House press secretary Sean Spicer said in Washington.

He said the Trump administration is taking steps to enhance its ability to defend against North Korea's ballistic missiles, such as through the deployment of a missile defense system. Seoul agreed with the Obama administration to place that system on its soil against the objections of China, which is concerned the system's radar will range inside its territory.

The New York Times reported over the weekend that the Obama administration also tried to conduct cyber and electronic strikes against North Korea's missile program.

Republican Sen. Cory Gardner, who chairs a Foreign Affairs subcommittee on Asia, told AP he has called for the administration to provide a closed briefing to senators. He said he wants clarity on what has been done and under what authorities, and what the U.S. posture toward North Korea will be in the months ahead.

He also stressed a need to clamp down on Kim's sources of foreign revenue and for China to follow through on its promise to suspend imports of North Korean coal.

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Associated Press writers Catherine Lucey in Washington and Edith M. Lederer at the United Nations contributed to this report. (**)