The United States on Thursday reassured its key Asian allies Japan and South Korea that its policy seeking North Korea's nuclear disarmament remains unchanged, after its intelligence chief called it a "lost cause".

Washington has always maintained it cannot accept North Korea as a nuclear state and, under President Barack Obama, has made any talks with Pyongyang conditional on the country first making some tangible commitment towards denuclearisation.

But in remarks to a think-tank earlier this week, US Director of National Intelligence James Clapper suggested that such a policy was based on wishful thinking, saying: "The notion of getting the North Koreans to denuclearise is probably a lost cause."

US Deputy Secretary of State Antony Blinken took issue with that view in Tokyo after a trilateral meeting with his Japanese and South Korean counterparts.

"Our policy has not changed," he told reporters at a joint news conference.

"We will not accept North Korea as a nuclear state, we will not accept North Korea's possession of nuclear weapons. Period."

The threat from North Korea is growing "more acute by the day" as the country ramps up its missile and nuclear tests, he added.

"We are focused on increasing the pressure on North Korea with one purpose: To bring it back to the table to negotiate in good faith denuclearisation. That is the objective," he said.

Agreeing with Blinken, Japanese vice foreign minister Shinsuke Sugiyama said the three countries "are closely cooperating" to implement existing UN Security Council sanction resolutions on Pyongyang and adopt a new sanction "that is meaningful."

The Security Council is currently discussing a new resolution to punish North Korea over its fifth nuclear test in September -- having already imposed tough economic measures after a fourth test in January.

Clapper's comment reflected an opinion widely held among North Korea experts but one only expressed in private by senior US administration officials who feel a policy change is overdue.

State Department spokesman John Kirby has already rebuffed Clapper's position, stressing that "nothing has changed" with the Obama administration's policy of pushing the North -- through a toughened sanction regime -- to give up its nuclear weapons.