LONDON — North Korea must be stopped on its path toward being able to threaten the United States with nuclear attack, U.S. Defense Secretary Jim Mattis said Friday in a stark expression of America’s top national security concern at the moment. He emphasized diplomatic means of changing Pyongyang’s “reckless” agenda.

On his first visit to Britain as Pentagon chief, Mattis also took rhetorical jabs at Russia and said America’s priority in Syria is defeating the Islamic State group rather than bringing down President Bashar Assad.

At a joint news conference with his British counterpart, Michael Fallon, Mattis was reminded by a reporter that as commander of U.S. forces in the Middle East several years ago, he considered Iran to be the biggest threat to U.S. interests. Asked how he would deal with Iran as secretary of defense, Mattis called Tehran a problem but quickly pivoted to condemning North Korea and described the isolated, communist country as the more immediate threat.

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