The United States is calling on North Korea to stick to its prior agreements on abandoning its nuclear weapons program. Delegates of six nations have opened a fifth round of negotiations on Pyongyang's nuclear ambitions in Beijing.

Speaking to reporters before the talks opened, U.S. negotiator Christopher Hill said the United States is standing firm on its expectations of North Korea. He said Pyongyang must rejoin the Non-Proliferation Treaty and submit to the rules of the International Atomic Energy Agency before it can receive the economic and energy aid promised to it.

U.S. Assistant Secretary of State Christopher Hill, left, arrives at talks with North Korean lead delegate Kim Gye Gwan

"Our delegation has made it very clear that first they have got to disarm, create a nuclear-free Korean Peninsula, and once they are back in the NPT, with IAEA safeguards, at an appropriate time we will have discussions on the subject of a light water reactor," he said.

In a joint statement following the last round of talks, North Korea agreed to give up its nuclear programs, and discuss a demand for a light water nuclear reactor later. But Pyongyang then backtracked and restated the demand.

The latest round of talks, which also include China, Russia and South Korea, are expected to last only a few days, dampening hopes for an immediate breakthrough.

Analysts say resolving the matter of "sequencing," or who takes which step and when, is perhaps the biggest hurdle that negotiators have to overcome at this stage. No one expects any quick action in a process that has been going on now for more than two years.

The negotiations aim to resolve a dispute that flared in 2002, when the United States said North Korea had admitted it was developing a uranium enrichment program, in violation of its international agreements. North Korea has since denied the existence of such a program, but says it is building a plutonium-based nuclear deterrent force to defend itself from a U.S. attack.

The Bush administration has repeatedly said it has no plans to attack North Korea, and says the six-party talks are the best hope for resolving the nuclear crisis peacefully.