Former US President Jimmy Carter urged the United States and South Korea Tuesday to talk directly with North Korea, saying a failure to negotiate nuclear disarmament might lead to a "catastrophic" war.

"No one can predict the final answers from Pyongyang, but there is no harm in making a major effort, including unrestrained direct talks," he said in a speech after receiving an honorary doctorate from Korea University in Seoul.

"The initiative must be from America and South Korea."

Carter made an unprecedented visit to Pyongyang in 1994 when the United States came close to war with North Korea over its nuclear programme. He helped defuse the crisis through talks with then-leader Kim Il-Sung.

Washington sent special envoy Stephen Bosworth to Pyongyang in December to try to persuade the North to return to stalled six-nation nuclear disarmament talks.

But it is cautious about holding more bilateral talks unless they lead directly to a resumption of the six-party forum -- grouping the two Koreas, the United States, China, Russia and Japan.

The North has set two conditions for returning to the six-party talks: the lifting of UN sanctions and a US commitment to discuss a formal peace treaty.

Washington, Seoul and Tokyo say the North must first return to the negotiating table and show it is serious about giving up its nuclear drive.

Carter, who arrived Sunday for a four-day visit, said the North would never back down unless the United States meets its basic demands such as direct talks leading to a simple framework for an agreement.

"First of all -- and difficult for South Korea and the United States -- is the need for more direct negotiations with North Korea," he said.

Washington should give a firm statement of "no hostile intent" among other actions to bring the North back to the negotiations.

"The alternative is a continuation of the present path of estrangement, isolation, additional suffering of innocent North Korean private citizens and ever-expanding conventional and nuclear arsenals, perhaps leading to a catastrophic war," he said.

Despite decades of economic sanctions and isolation, the North's regime is "relatively immune to further deprivation from embargoes", Carter said.

"I don't deny that some of this punishment has been merited, but it was obvious to me when I was in North Korea that there is deep resentment of the past and genuine fear of preemptive military attacks in the future," he said.

The former president also urged leader Kim Jong-Il to honour his late father's 1994 promise to give up nuclear weapons.

The North has conducted two atomic weapons tests, in 2006 and 2009. It also says it has a successful experimental enriched uranium programme, a second way to make a nuclear bomb.