A team of U.S. experts will begin disabling North Korea’s nuclear facilities Monday, the top U.S. envoy to nuclear disarmament talks with Pyongyang said, marking the biggest step the communist country has ever taken to scale back its atomic program.

Christopher Hill also said North Korea — one of the world’s most isolated countries — appeared to be opening up, and said efforts had begun towards removing the communist regime from Washington’s list of countries sponsoring terrorism.

“By Monday morning, they will begin their work,” Hill said, referring to the U.S. team that arrived in Pyongyang on Thursday. “It’s a very big day because it’s the first time it’s actually going to start disabling its nuclear program,” he said.