"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 that 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.