US Secretary of State John Kerry has warned North Korea it would be a "huge mistake" to launch a medium-range missile and said the United States would never accept the reclusive country as a nuclear power.

Speaking after talks with South Korea's president and leaders of the 28,000-strong US military contingent in the country, Mr Kerry also said it was up to China, North Korea's sole major ally, to "put some teeth" into efforts to press Pyongyang to abandon its nuclear ambitions.

Mr Kerry, like other US officials, played down an assessment from the Pentagon's intelligence agency that the North already had a nuclear missile capacity. The US, he said, wanted to resume talks about North Korea's earlier pledges to halt its nuclear programme, but would defend its allies in the region if necessary.

North Korea has repeatedly said it will not abandon nuclear weapons, which it said yesterday were its "treasured" guarantor of security.

Mr Kerry's visit coincided with preparations for Monday's anniversary of North Korean state founder Kim Il-sung's birth date, a possible pretext for a show of strength, with speculation focusing on a possible new missile launch.

Mr Kerry, who flies to China today and to Japan tomorrow, said that if North Korea's 30-year-old leader, Kim Jong-un, proceeded with a launch, "he will be choosing, wilfully, to ignore the entire international community".

He added: "I would say ahead of time that it is a huge mistake for him to choose to do that because it will further isolate his country and further isolate his people, who frankly are desperate for food, not missile launches."

The North has issued weeks of shrill threats of an impending war following the imposition of UN sanctions in response to its third nuclear test in February. Mr Kerry said the threats were "simply unacceptable" by any standard.

"We are all united in the fact that North Korea will not be accepted as a nuclear power," he said. Mr Kerry later told US businessmen in Seoul that China, as an advocate of denuclearisation, was in a position to press for a change in the North's policy.

"The reality is that if your policy is denuclearisation, and it is theirs as it is ours, as it is everybody's except the North at this moment - if that's your policy, you've got to put some teeth into it," he told the gathering.

But North Korea has shown little inclination for further talks.

Rodong Sinmun, the mouthpiece of the ruling Workers' Party, said Pyongyang would never abandon its nuclear programme.

"The DPRK will hold tighter the treasured sword, nuclear weapons," it said, referring to the country by its official name, the Democratic People's Republic of Korea.

A Chinese Foreign Ministry spokesman said Beijing sought "peace and stability on the Korean peninsula and [was] pushing for its denuclearisation via talks and consultations".