U.S. President Donald Trump said Thursday that North Korea has started to rid itself of nuclear weapons following an agreement with its leader Kim Jong Un earlier this month.

His comment, however, contradicted Defense Secretary Jim Mattis, who said Wednesday he had seen no signs that Pyongyang had taken concrete actions toward denuclearization.

"The big thing is, it will be a total denuclearization, which is already starting taking place," Trump said at a Cabinet meeting, part of which was open to the media.

"They've already blown up one of their big test sites. In fact, it was actually four of their big test sites," he said.

Trump was referring to North Korea's claim that it had destroyed its only known nuclear test site in Punggye-ri in the country's northeast. But it was not known what Trump meant by saying "four of their big test sites."

Asked by a reporter Wednesday if he had seen North Korea dismantling its nuclear weapons program, Mattis said, "I'm not aware of that. Obviously, it's the very front end of a process."

"The detailed negotiations have not begun," the Pentagon chief said. "I wouldn't expect that at this point."

[Getty/Kyodo]

On Thursday, Trump said Kim has stopped testing missiles, including ballistic missiles that could reach the United States, and is destroying the North's main engine testing site, in an apparent reference to the Sohae Satellite Launching Station in Tongchang-ri, northwestern North Korea.

However, commercial satellite imagery of the Sohae station from June 12 shows no apparent activity related to the dismantlement of its rocket engine test stand, according to 38 North, a U.S.-based group monitoring North Korea.

The engine test stand at Sohae has been used to test large liquid-fueled rocket engines applicable to both intercontinental ballistic missiles and large space launch vehicles.

During the first-ever U.S.-North Korea summit on June 12 in Singapore, Kim reaffirmed his "firm and unwavering commitment to complete denuclearization of the Korean Peninsula," while Trump committed to provide security guarantees to North Korea.

Speaking at the Cabinet meeting, Trump corrected the comment he made Wednesday that the remains of 200 U.S. troops missing in the 1950-1953 Korean War had already been sent back, in line with the agreement he made with Kim in Singapore.

"They've already sent back, or are in the process of sending back, the remains of our great heroes who died in North Korea during the war," he said. "And that's already in the process of coming back."

Trump also said Japanese Prime Minister Shinzo Abe was "thrilled" that Japan would no longer see North Korean missiles fly over Japan.

"There were plenty of them sent right over Japan, and he (Abe) said, 'I want to thank you because what you've done is incredible. There are no more rockets going, and there's no thought of it,'" the president said.

Trump said his relationship with Kim will "lead to tremendous success" in achieving denuclearization and ending decades of hostility between the two countries.