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 is in South Korea, where he has been assuring Seoul it has America’s continuing support.
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 maintaining that presence.
Mr Mattis has been using his visit to reassure South Korea that the Trump administration “remains steadfast” in its “iron-clad” defence commitments to the region, said the Pentagon.
Speaking after talks at the defence ministry with his South Korean counterpart Han Min-koo, Mr Mattis told reporters that “any attack on the United States, or our allies, will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming”.
North Korea conducted its fifth test of a nuclear device last year, and claims it is capable of carrying out a nuclear attack on the US, though experts are still unconvinced its technology has progressed that far.
It has also said in recent weeks that it has a new intercontinental ballistic missile, capable of reaching the US mainland, which it is prepared to test launch at any time.