Secretary-General Antonio Guterres says he believes war over North Korea's nuclear weapons is avoidable — but he's worried and "not yet sure that peace is guaranteed."

The U.N. chief says "there is a window of opportunity" that he hopes will "make the war avoidable, but it is important that we don't miss the opportunities that windows can provide."

Guterres told reporters Tuesday that's why the U.N. is "so strongly engaged" and why he sent U.N. political chief Jeffrey Feltman to Pyongyang in early December, which South Korea says contributed to restarting dialogue between the two Koreas.

The secretary-general said "there are some signals of hope" and it's extremely important to use them "to make sure that a serious process leading to the peaceful denuclearization of the Korean Peninsula takes place."