(Adds Japanese delegate's comments, paragraphs 11,12) By Chris Buckley BEIJING, Feb 7 (Reuters) - Delegates to six-party talks began converging on Beijing on Wednesday seeking a breakthrough in talks aimed at curtailing North Korea's nuclear ambitions, but envoys and analysts said a final deal was far away. Flying in to a city decked out for the upcoming Lunar New Year holiday, the U.S. chief negotiator said the talks starting on Thursday would focus on implementing a 2005 statement offering isolated North Korea economic and security concessions in return for ending its nuclear weapons aspirations. "But I want to emphasise that the real success is when we complete the September '05 agreement," Assistant Secretary of State Christopher Hill told reporters after arriving in Beijing."Not just when we start the '05 agreement, but when we finish it, so we're not going to finish that this week. We'll just maybe take a good first step," he said. China has set no official deadline for the session, but its chief delegate, Wu Dawei, has said the talks may last three or four days. The Lunar New Year of the Pig begins on Feb. 18. Efforts to convince Pyongyang to renounce nuclear arms have assumed fresh urgency since it defied international warnings and carried out a first nuclear test in October, prompting U.N. sanctions endorsed by even the North's long-time backer, China. Hill said progress at the latest round of the intermittent talks depended on all six parties -- host China, the two Koreas, the United States, Japan and Russia. "We worked very hard for this, done consultations all over the place, so let's see if we can make some progress," he said. South Korean envoy Chun Yung-woo and Japan's chief delegate, Kenichiro Sasae, echoed the need to make headway. "The six-party talks are at a moment of truth. At this round, we must turn a corner in making North Korea's pledge to de-nuclearise a reality," Chun told reporters. Sasae said the talks were a watershed. "It is crucial for us to secure concrete steps towards the de-nuclearisation of North Korea," Sasae told reporters. "To this end we want to coordinate with other countries concerned to produce results." CHINESE DOUBTS But a Chinese nuclear expert from an official thinktank voiced doubts about the readiness of Pyongyang and Washington to set aside their differences. "Frankly speaking, I have changed my view from optimistic to pessimistic," Teng Jianqun, deputy secretary-general of the China Arms Control and Disarmament Association, told a news briefing in Beijing. A U.S. financial crackdown on North Korea, as well as the North's nuclear test, stood in the way of potential agreement, said Teng, adding: "Mistrust between the two countries is the core element." "I am just wondering whether the parties concerned, especially the U.S. and North Korea, would like to solve the dispute in the short term. The answer might be not," he said. The latest round is likely to focus on persuading North Korea to first shut down its Yongbyon nuclear plant, a source of plutonium for its nuclear weapons programme. Washington has said that North Korea has also run a uranium enrichment programme, providing another potential source of weapons material. But the North has its own demands and they may again bog down negotiations as they did in December. North Korea has said it will not contemplate scrapping its nuclear weapons until Washington lifts a banking crackdown prompted by accusations that Pyongyang ran dollar counterfeiting and other illicit business. A new round of bilateral talks in Beijing in January between U.S. Treasury Department and North Korean officials seeking to resolve the dispute ended with no sign of a breakthrough. China has not set any official deadline for the six-party talks, but its chief delegate, Wu Dawei, has said they may last three or four days. (Additional reporting by Lindsay Beck and Jack Kim)
