The US Defence Secretary James Mattis has said any use of nuclear weapons by North Korea would be met with an "effective and overwhelming" response. <br/><br/>Mr Mattis spoke in South Korea, where he had been reaffirming US support, before flying to Tokyo. He also reconfirmed plans to deploy a US missile defence system in South Korea later this year. <br/><br/>North Korea's repeated missile and nuclear tests and aggressive statements continue to alarm and anger the region. <br/><br/>The US has a considerable military presence in South Korea and Japan, as part of a post-war defence deal. President Donald Trump has previously said he wants both South Korea and Japan to pay more towards maintaining that presence.<br/><br/>Mr Mattis used his visit to reassure South Korea that the Trump administration "remains steadfast" in its "iron-clad" defence commitments to the region.