US reassures Asian allies it is still seeking North Korean nuclear disarmament despite 'lost cause' claims

'We will not accept North Korea as a nuclear state, we will not accept North Korea's possession of nuclear weapons. Period,' US Deputy Secretary of State says

Kim Jong-Un's recent nuclear test caused great concern in South Korea (AP)

The United States has reassured Japan and South Korea it spolicy seeking North Korean nuclear disarmament remains unchanged, after its top intelligence official called it a "lost cause".

James Clapper, US Director of National Intelligence, suggested Washington's policy of making any talks with Pyongyang conditional on the country making a tangible commitment towards denuclearisation was based on wishful thinking.

"The notion of getting the North Koreans to denuclearise is probably a lost cause," he told the Council on Foreign Relations think-tank in New York on Tuesday.

However, US Deputy Secretary of State Antony Blinken took issue with his view. "Our policy has not changed," he told reporters at a joint news conference in Tokyo, after a trilateral meeting with his Japanese and South Korean counterparts.

"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."

Japan, the United States and South Korea agreed to work together to put more pressure on North Korea to abandon its nuclear and missile programmes.

"We reaffirmed the necessity to increase pressure against North Korea to have it give up its nuclear and missile development and realise the denuclearisation of the peninsula," Japanese Vice Foreign Minister Shinsuke Sugiyama said on Thursday.

Agreeing with Mr Blinken, Mr 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.

North Korea has insisted it will never give up its nuclear weapons and declared itself a "nucler state" in its 2012 constitution. One of the government's key policies, byungjin, calls for developing a nuclear arsenal even as it builds its domestic economy.

Pyongyang regularly makes atomic threats and recently warned it would reduce Seoul and Washington to "a heap of ashes through a Korean-style pre-emptive nuclear strike" it if detected any sign of aggression.

Additional reporting by agencies