Delegates to six-nation talks aimed at ending North Korea's nuclear weapons program opened a fifth round of negotiations Wednesday with the task of working out how to implement a disarmament pact signed in September.

China's chief negotiator, Deputy Foreign Minister Wu Dawei, began the session with an appeal for all parties to be flexible "so that we will be able to work out an implementation plan that is acceptable to all sides at an early date."

But North Korea's official Korean Central News Agency said the government was "deeply concerned" about the future of the talks because of a remark made by President Bush in Brazil on Sunday referring to "a tyrant in North Korea."

Quoting a North Korean Foreign Ministry spokesman, the agency condemned Bush's comments as a "blatant violation" of the September agreement and said they "deprive us of any trust in the negotiators of the U.S. side to the six-party talks."

The report underscored the differences between North Korea and the United States as their envoys and those from four other nations -- China, Japan, South Korea and Russia -- sat down to continue negotiations that began more than two years ago.

The last round ended in September with a general agreement in which North Korea promised to disarm in exchange for aid, diplomatic recognition and security guarantees. It is the only written accord reached by the negotiators.

But the deal left key disputes unresolved, including how and when the North Korean government of Kim Jong Il would dismantle its nuclear program, how the other nations would verify the dismantling, and when they would begin providing the North with the promised economic and diplomatic benefits.

Most notably, the parties agreed to discuss "at an appropriate time" providing North Korea with a light-water nuclear reactor. The Bush administration has said the only appropriate time for such a discussion would be after North Korea dismantled its nuclear facilities and allowed international inspections. But North Korea has demanded the reactor be built before it disarms.

China has declared that the talks will adjourn after three days to let the officials attend an Asia-Pacific economic forum in South Korea next week. Analysts and diplomats said little progress is expected; the parties planned to exchange proposals on how to proceed and resume discussions later this year.

Assistant Secretary of State Christopher R. Hill, the chief U.S. envoy, said the parties should first agree on what specific steps North Korea must take to disarm, suggesting that the questions of what benefits it would receive in return, and when, should be set aside.

But South Korea's chief delegate, Deputy Foreign Minister Song Min Soon, told reporters after a meeting with his North Korean counterpart that the Pyongyang government wanted to discuss when it would get the light-water reactor.

"North Korea is not going to give up its demand for a light-water reactor in this round and that it be discussed first, before disarming," said Shen Dingli, an international relations expert at Fudan University in Shanghai, who visited Pyongyang this year.

Last month, President Hu Jintao of China made a rare visit to Pyongyang and secured a promise from Kim to press ahead with the talks. Chinese analysts with close ties to the government said high-level visits between China and North Korea are usually accompanied by substantial packages of economic aid to the North, and Hu is believed to have pledged a multi-year increase in aid during the trip.

The aid may have persuaded North Korea to return to the talks but could also take pressure off it to make a deal. North Korea claims that its crippled economy is recovering. If true, analysts said, the North might conclude that it can afford to drag out talks while continuing to develop nuclear arms.