North Korea has thrown a major obstacle in front of an agreement it joined just one day earlier, by demanding the United States provide it with a civilian nuclear reactor before Pyongyang makes any moves toward disarmament.

South Korean television image shows North Korean light water nuclear reactor

A North Korean broadcaster reads a Foreign Ministry statement demanding the United States provide it with a light water nuclear reactor, before Pyongyang will return to the global Non-Proliferation Treaty, or readmit international nuclear inspectors.

Just a day earlier, North Korea had joined Russia, China, Japan, South Korea, and the United States in a joint statement of principles, which committed Pyongyang to dismantling its nuclear weapons and programs.

The six parties agreed to discuss the possibility of providing a civilian reactor to the North "at an appropriate time" - seemingly implying that North Korea would first make moves toward disarmament.

A senior U.S. State Department official reacted sharply to North Korea's latest statement Tuesday, saying it was not part of Monday's agreement. The Japanese Foreign Minister said there was consensus at the six-party talks that such a demand by Pyongyang would be "unacceptable."

South Korea, which has been lobbying hard on North Korea's behalf, sought to downplay the North's latest demand. Foreign Ministry Spokesman Lee Kyu-hyung, portrayed it as an attempt by Pyongyang to enhance its bargaining leverage when the talks resume in November.

"We assume that North Korea tries to demand at the maximum level," he said. " We believe the issue shall be discussed specifically at the forthcoming round of six-party talks."

But Mr. Lee also says Seoul's willingness to support peaceful nuclear energy use by North Korea depends on Pyongyang first rejoining the Non-Proliferation Treaty and bringing U.N. inspectors back.

The United States and a group of nations began building a light-water nuclear reactor for North Korea as part of a 1994 agreement, which froze the North's plutonium-based weapons program. Washington halted construction in 2002, when, it says, Pyongyang admitted to pursuing a secret uranium-based weapons program. Soon afterwards, North Korea evicted international nuclear inspectors, withdrew from the NPT, and restarted its plutonium program.

Peter Beck, of the research organization International Crisis Group, says he views North Korea's demand as a signal it is not serious about negotiating away its nuclear programs.

"For anybody in Pyongyang to think we can go back to 1994, I mean, even under the most liberal of U.S. governments is, you know, impossible," he said.

Experts note that the negotiations have yet to confront the dilemma of "sequencing," or who takes which steps and when. With very little trust between the United States and North Korea, neither side wants to make to too large a first step, for fear the other side will renege on its part of the bargain.