Photo: The Canadian Press

The Trump administration told lawmakers Wednesday it will apply economic and diplomatic pressure on North Korea to dismantle its nuclear weapons program, as an extraordinary White House briefing served to tamp down talk of military action against an unpredictable and increasingly dangerous U.S. adversary.

President Donald Trump welcomed Republican and Democratic senators before his secretary of state, defence secretary, top general and national intelligence director conducted a classified briefing. The same team was also meeting with House members in the Capitol to outline the North's escalating nuclear capabilities and U.S. response options to what they called an "urgent national security threat."

After weeks of unusually blunt military threats, the joint statement by the agency chiefs said Trump's approach "aims to pressure North Korea into dismantling its nuclear, ballistic missile and proliferation programs by tightening economic sanctions and pursuing diplomatic measures with our allies and regional partners." It made no specific mention of military options, though it said the U.S. would defend itself and friends.

The unprecedented meeting in a building adjacent to the White House reflected the increased American alarm over North Korea's progress in developing a nuclear-tipped missile that could strike the U.S. mainland. A flurry of military activity, by North Korea and the U.S. and its partners on and around the divided Korean Peninsula, has added to the world's sense of alert.

While tensions have increased since Trump took office, they've escalated dramatically in recent weeks as American and other intelligence agencies suggested the North was readying for a possible nuclear test. Trump has sent high-powered U.S. military vessels and an aircraft carrier to the region in a show of force, while the North conducted large-scale, live-fire artillery drills, witnessed by national leader Kim Jong Un, earlier this week.

On Wednesday, South Korea started installing key parts of a contentious U.S. missile defence system that also has sparked Chinese and Russian concerns.

America's Pacific forces commander, Adm. Harry Harris Jr., told Congress on Wednesday the system would be operational within days. He said any North Korean missile fired at U.S. forces would be destroyed.