Washington (AFP) - Convincing North Korea to abandon nuclear weapons is a "lost cause," America's top intelligence official said, causing concern in the State Department and ally South Korea over an issue of long-standing US policy.

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

But in remarks to the Council on Foreign Relations think-tank, US Director of National Intelligence James Clapper on Tuesday suggested such a policy was based on wishful thinking.

"The notion of getting the North Koreans to denuclearize is probably a lost cause. They are not going to do that. That is their ticket to survival," Clapper said.

"They are under siege, and they are very paranoid. So the notion of giving up their nuclear capability, whatever it is, is a nonstarter with them," he added.

- Policy change? -

His comments 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 on North Korea is overdue.

While Clapper may have been seeking to shore up arguments to support the imminent deployment of the US THAAD missile defense system in South Korea, his remarks add a high-profile voice to the growing debate over how the next US president should handle North Korea.

State Department spokesman John Kirby 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.

"We want to continue to see a verifiable denuclearization of the (Korean) peninsula," Kirby said.

Critics of the policy say sanctions and non-engagement have done nothing to prevent the North's accelerated drive towards a credible nuclear deterrent that could directly threaten the US mainland.

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- Steadfast South -

South Korea, which has stood shoulder-to-shoulder with the US hardline on Pyongyang, also stressed there would be no change of course.

"The determination of not only South Korea and the US but of the international community to end North Korea's nuclear program is stronger than ever," a foreign ministry official told AFP.

"We will work with the international community to impose stronger sanctions and pressure on the North so it will have no other choice but to denuclearize," the official said.

Although there is no official dialogue between Washington and Pyongyang, there are regular, so-called Track 2 discussions involving former US diplomats and North Korean officials -- most recently in Malaysia last weekend.

In July, the North cut off its only remaining official channel of diplomatic communications with the United States in retaliation for American sanctions against its leader, Kim Jong-Un.

North Korea has been hit by five sets of UN sanctions since it first tested a nuclear device in 2006.

After Pyongyang carried out its fourth nuclear test in January, the Security Council adopted the toughest sanctions resolution to date, targeting North Korea's trade in minerals and tightening banking restrictions.

Council members are currently debating a fresh resolution after the North's fifth nuclear test in September.

According to Security Council diplomats, the negotiations are focused on closing loopholes and zeroing in on North Korea's nuclear and ballistic missile technology industry.