WASHINGTON -- Secretary of State Rex Tillerson on Tuesday softened America's stance on possible talks with North Korea, calling it "unrealistic" to expect the nuclear-armed country to come to the table ready to give up a weapons of mass destruction program that it invested so much in developing. Tillerson said his boss, U.S. President Donald Trump, endorses this position.

Tillerson's remarks came two weeks after North Korea conducted a test with a missile that could potentially carry a nuclear warhead to the U.S. Eastern Seaboard -- a milestone in its decades-long drive to pose an atomic threat to its American adversary that Trump has vowed to prevent, using military force if necessary.

"We are ready to talk anytime North Korea would like to talk. And we are ready to have the first meeting without preconditions," Tillerson said at the Atlantic Council think-tank .

He said that the North would need to hold off on its weapons testing. This year, the North has conducted more than 20 ballistic missile launches and one nuclear test explosion, its most powerful yet.

"Let's just meet and we can talk about the weather if you want to. We can talk about whether it's a square table or a round table if that's what you are excited about," Tillerson said. "But can we at least sit down and see each other face to face and then we can begin to lay out a map, a road map, of what we might be willing to work towards."

Although Tillerson said the goal of U.S. policy remained denuclearization of the Korean Peninsula, he added it was "not realistic to say we're only going to talk if you come to the table ready to give up your program. They've too much invested in it. The president is very realistic about that as well."

White House spokeswoman Sarah Huckabee Sanders said in a statement later Tuesday that: "The President's views on North Korea have not changed."

"North Korea is acting in an unsafe way not only toward Japan, China, and South Korea, but the entire world. North Korea's actions are not good for anyone and certainly not good for North Korea," she said.

In public, Trump has been less sanguine about the possibilities of diplomacy with Kim Jong Un's authoritarian government, which faces growing international isolation and sanctions as it pursues nuclear weapons in defiance of multiple U.N. Security Council resolutions. In October, Trump appeared to undercut Tillerson when he said he was "wasting his time" trying to negotiate with the North Korea, just as Tillerson said the U.S. had backchannel communications with the North.

Trump, who has traded insults with Kim, kept up his tough talk on Tuesday. As he signed a $700 billion defence authorization bill that includes additional spending on missile defence, he referred to North Korea as a "vile dictatorship."

"We're working very diligently on that -- building up forces. We'll see how it all turns out. It's a very bad situation -- a situation that should have been handled long ago by other administrations," Trump said.

Tillerson did not indicate that North Korea had signalled a new readiness to talk, but said that "they clearly understand that if we're going to talk, we've got to have a period of quiet" in weapons tests.

Tillerson stressed that the U.S. would not accept a nuclear-armed North Korea, as it flouts international norms and might spread weapons technology to non-state groups in ways that other nuclear powers have not.

In a rare admission of discussion of a highly sensitive topic, Tillerson said Washington has discussed with Beijing how North Korea's nuclear weapons might be secured in case of instability there.

"The most important thing to us would be securing those nuclear weapons that they have already developed and ensuring that nothing falls into the hands of people who we would not want to have it. We've had conversations with the Chinese about how that might be done," Tillerson said.

It appeared to the first public recognition from an administration official that the U.S. has discussed North Korean contingencies with China, which fought with the North against the U.S. in the 1950-53 Korean War. The Trump administration has held a series of high-level dialogues with Beijing this year, and U.S. and Chinese generals held rare talks in late November about how the two militaries might communicate in a crisis although U.S. officials said the dialogue wasn't centred on North Korea.

Tillerson said that the U.S. has assured China that in the event that American troops had to cross northward of the demilitarized zone separating the two Koreas, it would retreat back south once stability returned.

"That is our commitment we made to them. Our only objective is to denuclearize the Korean Peninsula, and that is all," Tillerson said.

Daryl Kimball, executive director of the Arms Control Association, said Tillerson's proposal for direct talks with North Korea without preconditions was overdue and a welcome shift in position, but both sides needed to demonstrate restraint.

"For North Korea that means a halt to all nuclear and ballistic missile tests, and for the United States, refraining from military manoeuvrs and overflights that appear to be practice runs for an attack on the North," Kimball said. "If such restraint is not forthcoming, we can expect a further escalation of tensions and a growing risk of a catastrophic war."

Last week, the United States flew a B-1B supersonic bomber over South Korea as part of a massive combined aerial exercise involving more than 200 warplanes. North Korea says such drills are preparations for invasion.