The nuclear-armed North has issued increasingly strident declarations in recent weeks, even promising an ominous "Christmas gift" if Washington does not come up with some concessions.

The top US envoy to the negotiations with North Korea, Stephen Biegun, said in Seoul on Monday that Pyongyang's rumblings were "hostile and negative" -- and Trump said he was watching.

"We'll see. I'd be disappointed if something would be in the works. And if it is, we'll take care of it," Trump said at the White House when asked about the situation. "We're watching it very closely."

The negotiations between Washington and Pyongyang have been largely stalled since the collapse of a February summit in Hanoi between Trump and North Korean leader Kim Jong Un.

The North has said that if Washington fails to make it an acceptable offer, it will adopt a so far unspecified "new way."

It has carried out a series of tests at its Sohae rocket facility this month, after a number of weapons launches in recent weeks.

"It's a concern -- their rhetoric," US Defense Secretary Mark Esper told reporters, saying further action by North Korea could be likely.

Japan and others have said recent launches involved ballistic missiles, which Pyongyang is banned from testing under UN sanctions.