In written testimony to lawmakers ahead of a hearing on the Pentagon budget, Mattis said North Korea is increasing the pace and scope of its nuclear weapons program that leader Kim Jong-un has stated will one day be capable of delivering a bomb on the United States.
"The most urgent and dangerous threat to peace and security is North Korea," Mattis said.
"The regime's nuclear weapons program is a clear and present danger to all, and the regime's provocative actions, manifestly illegal under international law, have not abated despite United Nations' censure and sanctions."
The defense secretary also warned of a return to "Great Power competition," where countries like Russia and China are gaining military assertiveness and placing long-held global security protocol at risk.
"Both Russia and China object to key aspects of the international order so painstakingly built since the end of World War II," he said.
Pyongyang has test-fired a string of missiles this year, building on launches and atomic tests that have ratcheted up tensions over its quest to develop weapons capable of hitting the United States - something President Donald Trump has said "won't happen."
Mattis and his top military officer, General Joe Dunford, said any military action against North Korea would have disastrous consequences for the peninsula.
"It would be a war like nothing we have seen since 1953," Mattis said, pointing to the end of the Korean War.
"It would be a very, very serious war."
