“Right now, it appears to be going in a very reckless manner in what its conduct is portraying for the future and that has got to be stopped,” he told a press conference in London after meeting with his British counterpart, Sir Michael Fallon, according to NBC News.

The defense secretary’s comments come as North Korea has been conducting missile tests under dictator Kim Jong-Un. The reclusive country’s nuclear program has advanced recently and seems to be moving towards the ability to hit the U.S., according to the Associated Press.

Mattis added that the North Atlantic Treaty Organization (NATO), seen as an international safeguard against North Korea nuclear attacks, stood united.

“NATO stands united, the transatlantic bond is united, we are going to maintain article 5 as absolute bedrock of the NATO alliance,” he said.