U.S. Secretary of Defense Robert Gates says he’s surprised and disturbed by the rhetoric coming out of North Korea.

Gates made the comment during a news briefing on Tuesday after being asked to comment on North Korea’s threat that it would regard South Korea’s participation in the U.S.-led Proliferation Security Initiative (PSI) as a declaration of war.

Gates added that the communist state is isolating itself further with such comments.

The PSI is a set of initiatives that aims to halt the spread of weapons of mass destruction.