The United States said Saturday it was "encouraged" by surprise talks between North and South Korea over Pyongyang's nuclear programme, but remained cool on resuming disarmament talks.

US Secretary of State Hillary Clinton told regional foreign ministers at an Asian security forum in Indonesia that the onus remained squarely on the North to prove its sincerity before the stalled six-party talks could resume.

"We are encouraged by the recent North-South meeting that took place on the margins of the ASEAN Regional Forum, but we remain firm that in order for six-party talks to resume, North Korea must take steps to improve North-South relations," she said in prepared remarks.

In a joint statement released later Saturday, the United States, South Korea and Japan also said Pyongyang must "address" its secretive uranium enrichment programme before the talks, in limbo since December 2008, could re-start.

The allies issued a joint statement after their foreign ministers met on the Indonesian island of Bali saying the uranium issue was among the key obstacles to the resumption of the six-nation dialogue.

"The ministers ... agreed that North Korea's uranium enrichment programme must also be addressed in order to allow for the resumption of the six-party talks," they said.

The statement came after surprise meetings between the two Koreas' senior nuclear envoys in Bali on Friday and their foreign ministers on Saturday, in which both sides spoke of the need for the six-party process to resume.

The six-nation talks, involving the two Koreas, China, the United States, Japan and Russia, are aimed at convincing the North to give up its nuclear programme in return for diplomatic and economic rewards.

But in April 2009 Pyongyang formally abandoned the six-party talks, a month before staging its second atomic weapons test. In September of that year it announced it had reached the final stages of enriching uranium.

The North, using plutonium extracted from its Yongbyon reactor, conducted its first nuclear test in 2006. It is believed to have enough plutonium for six to eight atomic bombs.

The North then revealed an apparently operational uranium enrichment plant at its Yongbyon atomic complex to visiting US experts on November 12 last year.

Pyongyang says its new operation is intended to fuel a nuclear power plant, but senior US and other officials fear it could easily be reconfigured to produce weapons-grade uranium to augment the country's plutonium stockpile.

The United States said in December that North Korea had "at least one other" uranium enrichment site than that disclosed to the US experts, saying the issue raised concerns.

This week's rare and unexpected contacts between senior officials from the two Koreas gave fresh hope that the conditions had been met for a resumption of the six-party dialogue.

South Korean nuclear envoy Wi Sung-Lac and his counterpart from the North, Ri Yong-Ho, met for more than two hours at a luxury hotel in Bali on Friday, on the sidelines of a week of wide-ranging talks among Asia-Pacific ministers.

Both emerged saying they hoped to re-start stalled six-party talks.

The South's foreign minister, Kim Sung-Hwan, then briefly met his North Korean counterpart, Pak Ui-Chun, on Saturday morning ahead of a regional security dialogue, according to South Korea's Yonhap news agency.

But the joint US-Japan-South statement made clear the Bali talks were only the beginning and "sincere and constructive inter-Korean dialogue" was essential before the wider multinational contact group could reconvene.

"The ministers welcomed the inter-Korean dialogue on denuclearisation held in Bali ... and they emphasised that the inter-Korean dialogue should be a sustained process going forward," they said.

Dongguk University professor Kim Yong-Hyun told AFP in Seoul the North-South meetings in Bali were a "first step toward the six-party talks", but emphasised that the uranium issue was a "very serious obstacle".