North Korea is a grave threat “that’s got to be stopped” and the US is working to do so through the United Nations and with close allies, Defense Secretary James Mattis said Friday during his first visit to the United Kingdom in his current position. Speaking alongside UK Defense Secretary Michael Fallon, Mattis said North Korea is a “threat of both rhetoric and growing capability.” North Korea recently has completed a litany of tests focusing on both its nuclear capability and its ballistic missiles. The country is reportedly preparing for another nuclear test, its sixth, based on recent satellite imagery showing heightened activity at an underground testing site, according to The Was?hington Post. Mattis said the US is g?oing through the UN and working diplomatically “with those that we might be able to enlist in this effort to get North Korea under control.”