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.
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.
North Korea's repeated missile and nuclear tests and aggressive statements continue to alarm and anger the region.
The US has a considerable military presence in South Korea and Japan, as part of a post-war defence deal. There are just under 28,500 US troops in the country, for which Seoul pays about $900m (£710m) annually.
President Donald Trump has previously said he wants both South Korea and Japan to pay more towards...Read more...