Any use of nuclear weapons by North Korea will be met with an "effective and overwhelming" response, US defence minister Jim Mattis has said.
He made the warning in Seoul during an appearance with his South Korean counterpart, Han Min Koo.
"North Korea continues to launch missiles, develop its nuclear weapons programme, and engage in threatening rhetoric and behaviour," Mr Mattis said.
"We stand with our peace-loving Republic of Korea ally to maintain stability on the peninsula and in the region.
"America's commitments to defending our allies and to upholding our extended deterrence guarantees remain ironclad.
"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."
US defence secretaries have long offered assurances to South Korea and Japan that its nuclear "umbrella" will protect them.
But the comments by Mr Mattis were tougher than usual and showed why he chose South Korea for his first visit overseas since becoming defence secretary on January 20.
He sought to reassure South Koreans of the long-standing US commitment to a defence treaty that President Donald Trump suggested during the campaign was a bad deal for America.
Mr Trump has complained the treaty disadvantaged the US and he would not rule out abandoning signatories if they refuse to pay more for their defence.
He also said that South Korea and Japan, which are already within the range of North Korean missiles, might acquire their own nuclear weapons rather than rely on US deterrence.
Last month, Mr Trump fuelled speculation of possible US military action to pre-empt North Korea's weapons development.
In response to threats by North Korea's leader Kim Jong Un, the president tweeted: "It won't happen!"
Mr Mattis also discussed with the South Koreans a timetable for deploying an advanced US missile defence system called the Theatre High-Altitude Area Defence, or THAAAD.
It is meant to improve protection of South Korea and Japan - as well as US troops stationed in both countries - against a North Korean missile attack.
Speaking to reporters as he travelled to South Korea, Mr Mattis said he would ask South Korean and Japan whether the current strategy for getting the North to curtail or eliminate its nuclear and missile programmes was adequate.
North Korea has said it is nearly ready to test-launch a ballistic missile of intercontinental range, meaning it could theoretically threat the United States.
The current caretaker South Korean government has said it supports THAAAD deployment, but the decision is in doubt because of approaching national elections.
Mr Han said North Korea's nuclear and missile programmes "blatantly threaten" the international community and have created a "severe security situation" on the Korean peninsula.
He described the visit by Mr Mattis so early in his tenure as a warning to North Korea.
After a series of meetings in Seoul, Mr Mattis was scheduled to fly to Tokyo on Friday for further consultations.
The US has 28,500 troops permanently based in South Korea and about 50,000 in Japan.