SEOUL: The United States and North Korea have a "common understanding" on the need to resume stalled nuclear disarmament negotiations, US envoy Stephen Bosworth said Thursday after a visit to Pyongyang.

But Bosworth said it was unclear when the North would return to the six-party forum which it quit in April, a month before staging its second nuclear test.

His three-day visit was the first official contact between Washington and Pyongyang since President Barack Obama took office in January, pledging direct diplomacy with America's adversaries.

"There was a common understanding with the DPRK (North Korea) on the need to implement the 2005 joint statement and resume the six-party process," Bosworth told a press conference on his return to South Korea.

"It remains to be seen when and how the DPRK will return to the six-party talks. This is something that will require further consultations among all six of us."

The talks group the two Koreas, the United States, Japan, China and Russia.

The US envoy termed his visit "very useful" and said the two sides exchanged views "in a candid and businesslike fashion".

In a six-party joint statement in September 2005, the North vowed to scrap its nuclear weaponry in exchange for aid, diplomatic benefits and talks on a permanent peace pact for the peninsula.

Bosworth said the two sides had no discussions on follow-up talks. "It is important to point out that these were exploratory talks, not negotiations," he said.

"It is certainly our hope based on these discussions in Pyongyang that the six-party talks can resume expeditiously and that we can get back to the important work of denuclearisation."

Bosworth said he held talks with Vice Foreign Minister Kang Sok-Ju and top nuclear envoy Kim Kye-Gwan, but did not seek a meeting with leader Kim Jong-Il or deliver any message from Obama.

"In fact, I am the message," he said.

The envoy said he told the North the United States remains committed to full implementation of the joint statement, including the establishment of a peace pact to replace the armistice which ended the 1950-53 war.

Analysts believe Pyongyang's main goal is to negotiate a peace treaty with Washington, which says this must be discussed within the six-party format.

"As President Obama has made clear, the United States is prepared to work with our allies and partners in the region to offer North Korea a different future (in return for denuclearisation)," he said.

Bosworth's trip capped a turbulent year. The North in April declared the six-party talks "dead" after international censure over its launch of a long-range rocket. It later said it had resumed making weapons-grade plutonium.

In May it staged its second nuclear test and followed up with a series of missile launches in July, attracting tougher UN sanctions.

Later in the year the North began striking a softer note in what some analysts saw as a bid to soften the sanctions.

In October it told key ally China it was ready to return to the six-nation talks, but only if direct dialogue with the United States proved satisfactory.

Obama has offered the isolated and impoverished nation security and prosperity if it honours its commitment to give up nuclear weapons.

But a senior US official warned this week it could expect continued "very strong enforcement" of sanctions if it continues shunning the six-party forum.

Yang Moo-Jin, of Seoul's University of North Korean Studies, said Bosworth made "meaningful progress" despite failing to set a date for the North's return to negotiations.

"Both sides have reached a common understanding of the usefulness of the six-party process," he told AFP. "I'm positive that the six-party talks will eventually resume."