Secretary of State Rex Tillerson expressed confidence Tuesday that Washington is doing all it can to force North Korea to negotiate its nuclear disarmament.

But -- even as he vowed to pursue his diplomatic strategy "until the first bomb drops" -- he also warned that the US military stands ready to act if necessary.

And he added that if diplomacy and sanctions fail and the fighting erupts, it will have been a personal failure of his efforts to resolve the crisis.

"So we need the DPRK to come to the table for talks. We're ready to talk any time they'd like to talk," he said in a speech to the Atlantic Council policy forum, referring to North Korea by its official acronym.

"But they have to come to the table, and they have to come to the table with a view that they need to make a different choice."

Tillerson insisted Pyongyang must show it is ready to consider surrendering its bombs.

The United States has mobilized the world community to impose stringent economic and diplomatic sanctions on Kim Jong-un's regime to halt its nuclear drive.

But Pyongyang has continued to test ever more powerful ballistic missiles and boasts it can now target the US mainland with its nuclear arsenal.

Washington has pledged to deliver a denuclearized Korean peninsula, but the strategy hinges on China maintaining pressure and Kim agreeing to talk.

"We want them to make the right choice, which is to stop and say: 'Let's sit down to talk about it,'" Tillerson said in an earlier speech to staff.

"Because if they keep going, they can cross a point at which there's nothing left for us in the diplomatic community to do," he warned.

"We've done everything we can do, as we don't want to get to that point," Tillerson added, stressing that he works closely with US Secretary of Defense Jim Mattis.

"And I've said to my partner Secretary Mattis many times: 'If we get there, I've failed. And I don't want to fail,'" he promised.

Later in the day, Tillerson returned to the theme in the speech to the Atlantic Council.

"Because of the situation, the president has ordered our military planners to have a full range of contingencies available and they are ready," he said.

"As I've told people many times, I will continue our diplomatic efforts until the first bomb drops.

"I'm going to be confident that we're going to be successful, but I'm also confident that Secretary Mattis will be successful if it ends up being his turn."