"We fully know about the sites being discussed, nothing new -- and nothing happening out of the normal," Trump wrote on Twitter.

"I will be the first to let you know if things go bad!" he said.

Researchers at the Center for Strategic and International Studies, a prominent Washington think tank, on Tuesday said that satellite imagery had found 13 missile bases undeclared by North Korea.

The bases can be used to hide mobile, nuclear-capable missiles, the study said, warning that North Korea could preserve the sites -- and the ability to attack -- even as it negotiates with Trump on a potentially landmark accord.

Trump described a report on the findings by The New York Times as "inaccurate" and "fake news."