WASHINGTON (Reuters) - The U.S. military said on Thursday that it had not increased the already high-level of vigilance or changed its posture on North Korea after President Donald Trump called off a historic summit with North Korean leader Kim Jong Un.

"There is a high state of vigilance - the state of vigilance that we always have because they have proven to be unpredictable in the past," Lieutenant General Kenneth McKenzie, joint staff director, told a news briefing at the Pentagon.

"But it is not a heightened state of vigilance. It is the normal state of vigilance that we maintain."

Trump canceled the summit a few hours after North Korea said it followed through on a pledge to blow up tunnels at its main nuclear test site, which Pyongyang said was proof of its commitment to end nuclear testing.

McKenzie said the U.S. military was still assessing the damage.

"We're looking at pictures of it right now and we don't have a final assessment," he said. "They obviously did some visible destruction of (the) entrance to the tunnel."

The announcement came as U.S. Defense Secretary Jim Mattis and other top U.S. military commanders gathered at Peterson Air Force Base in Colorado for a ceremony to usher in a new commander to oversee U.S. forces in North America, whose responsibilities include defending against an incoming missile attack.

The Pentagon acknowledged that Mattis knew about Trump's decision ahead of his public announcement.

But it was not immediately clear whether the two spoke ahead of time, how long they spoke or whether Mattis provided any direct input ahead of Trump's decision.

Trump, in public remarks, noted he had spoken with Mattis and other U.S. military commanders, adding "we are more ready than we have ever been before" for a potential conflict with North Korea.

McKenzie said there had been no change in the U.S. military's support to a U.S.-led campaign to pressure the North to denuclearize.

He said the military was ready to respond to any North Korean provocations, but did not speculate further.

"We'll see what develops over the next few days, if any provocative actions occur from DPRK, we will certainly, in concert with our allies and partners in the region, by ready for it," he said.

(Reporting by Idrees Ali and Phil Stewart; Editing by Bill Trott and Marguerita Choy)

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