SEOUL - The U.S. envoy for North Korea failed to secure a firm commitment from the isolated state to resume nuclear disarmament negotiations but said on Thursday he had won assurance that it supported the languishing deal.

Stephen Bosworth, speaking after a three-day trip to Pyongyang, described as candid and businesslike his talks with First Vice Foreign Minister Kang Sok-ju, the man seen as the mastermind of the North's nuclear policy.

(There) is common understanding with the DPRK (North Korea) on the need to implement the 2005 joint statement and to resume the six-party process, he said, referring to a 2005 deal where the North takes apart its nuclear arms programme in exchange for massive aid and an end to its diplomatic isolation.

But he added: It remains to be seen when and how the DPRK will return to the six-party talks.

North Korea walked away from the negotiations with China, Japan, Russia, South Korea and the United States a year ago.

Five months later it detonated a nuclear device, its second such test, earning it tightened U.N. sanctions which further damaged its crippled economy.

Bosworth, who was speaking in the South Korean capital, flies to Beijing on Friday, then to Tokyo and Moscow for talks before returning to Washington next week.

The resumption of talks with the North is something that will require further consultation from all six of us, he said.

It is important to point out that these were exploratory talks, not negotiations. It is certainly our hope, based on these discussions in Pyongyang, that six-party talks can resume expeditiously and we can get back to the important work of denuclearisation, Bosworth said.

Bosworth met top nuclear negotiator Kim Kye-gwan as well as Kang, leader Kim Jong-il's right-hand man for dealings with the outside world.

He said he conveyed President Barack Obama's message that failure to move ahead on the disarmament deal was an obstacle to realising Pyongyang's long-sought goal of improving ties with Washington.

Once the six-party process resumed, Bosworth said, progress could then also be made on issues such as talks on a formal peace treaty to end the 1950-53 Korean War officially.

Analysts said the North's broken economy may be forcing it back to the bargaining table, where it hopes to win aid in return for a fresh promise to give up its nuclear arsenal.

Pyongyang's recent move to revalue its currency may be a further indication that the communist state is struggling to maintain control of its economy, experts said.

The move appears to have been met with widespread anger after it inflated the price of goods for already impoverished consumers and slashed the wealth of a burgeoning merchant class.

Traders in the Chinese border city of Dandong told Reuters that business has been drying up in North Korea following the removal of two zeroes from the face value of the currency, a move that could spell months of uncertainty for the fragile economy.

(Additional reporting by Christine Kim, Jon Herskovitz in Seoul, Lucy Hornby in Dandong; Writing by Jonathan Thatcher; Editing by Paul Tait)