The U.S. ambassador to Japan says that North Korea remains the most immediate concern because of both its military and a possible regime collapse.

According to Japan’s Yomiuri Shimbun daily, U.S. ambassador to Tokyo John Roos said in a lecture at Waseda University Friday that the possibility of a regime collapse particularly in the context of leadership succession in the North is a growing concern.

Roos said that a North Korea that falls into internal disarray would pose monumental security challenges to Northeast Asia.

He said the U.S. military presence in Japan is important to deal with such risks as North Korea and China with its military modernization.

The diplomat said that the U.S. Marine Corps’ Futenma Air Station in Okinawa is one of the most critical of the forces the U.S. deployed in both peacetime and in the unlikely event of conflict. He added that the relocation of the marine unit will affect the mobility and efficiency of U.S. forces stationed in Japan.