The US stance, which appeared to signal a willingness to exhaust non-military options, was put forward in a statement following an unusual White House-hosted briefing for the entire Senate.
It described North Korea as "an urgent national security threat and top foreign policy priority."
"The United States seeks stability and the peaceful denuclearisation of the Korean peninsula.
"We remain open to negotiations towards that goal," it said in a joint statement by Secretary of State Rex Tillerson, Defense Secretary Jim Mattis and Director of National Intelligence Dan Coats.
"However, we remain prepared to defend ourselves and our allies."
Although the statement emphasised diplomacy, Democratic Senator Christopher Coons told reporters after the White House briefing that military options were discussed.
"It was a sobering briefing in which it was clear just how much thought and planning was going into preparing military options, if called for, and a diplomatic strategy that strikes me as clear-eyed and well proportioned," he said.
