North Korea must "understand the true cost" of clinging to nuclear weapons ambitions, but not cut from talks, Washington's top nuclear negotiator with the country said on Tuesday.
"Some people doubt the point of negotiating. They say, 'how can you talk to these terrible people?'" Christopher Hill said in an address at the Asia Society in New York.
"The format, we think, is working. The problem I think remaining is whether North Korea, taking the plutonium they've already produced, are really prepared to give up that plutonium."
Five states are involved in negotiations with North Korea – South Korea, the United States, China, Japan and Russia.
Hill, reportedly a leading candidate to be the next US ambassador to Iraq, said North Korea must "understand the true cost" of holding on to the approximately 30 kilogrammes of plutonium Pyongyang claims to have ready to use.
That cost, he said, includes preventing a peace treaty on the Korean peninsula and blocking access for the impoverished communist state to the global economy and development funds.
Hill criticised the North Koreans as "complete momentum killers" during six-nation negotiations aimed at dismantling the nuclear programme.
But he said negotiations should be stepped up, with North Korea reassured that full disablement of its nuclear plans would open the way to bilateral talks "leading to normalisation of relations."
He stressed that firmness is required with North Korea and that the five other six-party partners have a strong hand.
"I think we ought to be able to get this done," Hill said. "A country with such belligerent attitudes to its neighbours... needs to be dealt with firmly with respect to its nuclear weapons. I just can't imagine we'd ever accept it."
Hill indicated he also favours the approach by new President Barack Obama on exploring direct talks with Iran, a sharp change of policy from his predecessor George W Bush.
"I have found it is much better to make sure your interlocutor, sometimes your adversary, understands what your views are and I have found the best way to do this is directly," Hill said.
"It's much better to have face to face [discussion] and therefore be in their face and make them understand very clearly what you're trying to do."