"We are in a boxer stance, we are ready to respond," Lieutenant General Kenneth McKenzie, director of the US military's Joint Staff, told reporters.

"We'll see what develops over the next few days. If any provocative actions occur from (North Korea), we will certainly, in concert with our allies and partners in the region, be ready for it."

Secretary of State Jim Mattis spoke with Trump by phone earlier in the day, but officials would not say if he had been part of the decision process to scrap the summit or was merely informed of it.

Pentagon spokeswoman Dana White said the Pentagon is part of the US "maximum pressure" campaign against North Korea that includes tough sanctions, diplomatic moves and military readiness.

"We are ready to fight tonight. That's always been the case," she said at a joint briefing with McKenzie.

McKenzie added the Pentagon maintains a "very high" state of vigilance with North Korea.

"We will continue that going forward to include our missile defense activities," he said.

In a letter to Kim, Trump announced he would not go ahead with the high-stakes meeting set for June 12 in Singapore.

The letter spoke of America's "massive and powerful" nuclear capabilities, marking a sudden return to the bellicose rhetoric that had been characteristic of Trump until he first announced the summit.

Tensions with North Korea soared last year after Kim test-fired intercontinental ballistic missiles with a big enough range to hit the US, and conducted his sixth nuclear test.

McKenzie said the Pentagon had not changed anything militarily during the short-lived thaw.

"We didn't ramp up or down as word of this summit began to rise and now has ended," he said.

"We are being very steady, very straight in terms of our preparations, in terms of the readiness that we display."

The US military has some 28,500 troops based in South Korea and the two militaries routinely conduct joint training exercises on the peninsula.

The US has also deployed a THAAD anti-missile battery in South Korea, designed to shoot down any rockets from the North.