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.
"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 U.S. 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.
"We are working diplomatically, including with those that we might be able to enlist in this effort to get North Korea under control," he said. "But right now it appears to be going in a very reckless manner."
"That's got to be stopped," he concluded.
Mattis made clear he still worries about Iran's involvement in what the U.S. 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." He was referring to its annexation of Ukraine's Crimea region in 2014 and "mucking around insider other people's elections and that sort of thing." He also cited Russian outreach to Afghanistan's Taliban as a concern.
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.
Britain's Fallon said he and Mattis discussed the INF issue in their private talks Friday. More broadly, he said the NATO alliance cannot return to business as usual with Russia.
"There is a pattern of interference by Russia," Fallon said. When the West does engage with Moscow, he added, "we need to be wary of what Russia is up to."
On Syria, which has been wracked by years of civil and IS' insurgency, Mattis said the U.S. government is "working this one day at a time." He said the focus is IS' stated intention of attacking the West, including Europe.
"We're going to have to keep them on their back foot, and that's where we're concentrating at this point," he said, alluding to U.S.-supported offensives in Mosul, Iraq, and Raqqa, Syria.
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