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 US forces in the Middle East several years ago, he considered Iran to be the biggest threat to US interests.
"This is a threat of both rhetoric and growing capability," Mattis said, alluding to the North's recent progress in building nuclear bombs and developing an intercontinental ballistic missile to deliver such weapons to US soil.
Experts believe North Korea will develop such capability in the next years, despite an array of international sanctions on the country. The Trump administration has been conducting a broad policy review of North Korea that includes military options, but Mattis stressed other approaches.
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"That's got to be stopped," he concluded.
Mattis made clear he still worries about Iran's involvement in what the US sees as destabilizing activities across the Middle East. But he suggested that he now sees the world through a wider lens and that makes North Korea the more urgent problem.
He also aimed strong criticism at Russia, saying its "violations of international law are now a matter of record."
Mattis hinted the Trump administration was close to deciding how to respond to Russia's recent deployment of a ground-based medium-range cruise missile an action the Pentagon recently declared a violation of the 1987 Intermediate-range Nuclear Forces treaty.
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