"We fully know about the sites being discussed, nothing new -- and nothing happening out of the normal," Trump, who is seeking a second summit with North Korean leader Kim Jong Un, 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."

South Korea also earlier played down the study, saying that the sites had been known for years.

Kim Eui-kyeom, spokesman for South Korea's dovish president, Moon Jae-in, disputed that North Korea was being deceptive as Pyongyang had never promised to get rid of short-range missiles.

The CSIS report said that the bases were scattered around North Korea and at times in narrow mountain valleys, meaning they could be quickly moved to launch strikes.

CSIS expert Victor Cha, who was a top adviser to former president George W. Bush, said the report underscored the risk of Trump accepting a "bad deal" in which North Korea only dismantles its most visible weapons infrastructure.

Trump has declared himself "in love" with Kim after a first-ever summit between the two nations' leaders held in June in Singapore.

Secretary of State Mike Pompeo has visited North Korea four times this year in hopes of preparing an agreement, in which the United States could formally declare an end to the 1950-53 Korean War -- a longtime goal of the dynastic Kim regime as it seeks to safeguard its survival.

Trump, however, told a news conference last week that he was in "no rush" on North Korea after a senior delegation abruptly canceled a meeting planned with Pompeo in New York.

North Korea has boasted of its missile prowess and said that it can hit the continental United States, although many experts are skeptical of the claim.

Less disputable is that North Korea could quickly assault South Korea, including its capital Seoul, and Japan in a crisis.

North Korea deploys medium-range missiles just 55 to 100 miles (90 to 150 kilometers) from the Demilitarized Zone that divides the peninsula.