North Korea has said it will not scrap its nuclear programme until it is given a civilian nuclear reactor, undermining an agreement reached 24 hours earlier.
The Yongbyon plant has been a centre of controversy
Pyongyang agreed on Monday to dismantle its nuclear programme in return for aid and security guarantees, following six-nation talks.
The BBC's Charles Scanlon says that the North's new statement looks like a recipe for continued deadlock.
Both Japan and the US have rejected Pyongyang's demand for a reactor.
Japanese Foreign Minister Nobutaka Machimura said the North's demand was "unacceptable", Kyodo News agency reported.
Sean McCormack, a spokesman for the US State Department, said: "This was obviously not the agreement they signed, and we will see what the coming weeks bring."
South Korean Unification Minister Chung Dong-young said the issue would be discussed before the next round of talks, which is set to take place in November.
But diplomats involved in these negotiations have few illusions about the scale of the task facing them, our correspondent says.
At the end of the six-nation talks in Beijing on Monday, North Korea agreed to a statement of principle under which it would abandon all nuclear weapons and current nuclear programmes and return to the nuclear non-proliferation treaty.
DETAILS OF DEAL
N Korea to abandon all nuclear weapons and programmes
N Korea to return to nuclear treaty and UN monitoring
US states it has no intention of attacking N Korea
N Korea says it has right to "peaceful uses of nuclear energy"
N Korea's demand for light water reactor to be discussed at "appropriate time"
In return, Pyongyang was offered electricity and an assurance that the US "has no intention to attack or invade [North Korea] with nuclear or conventional weapons".
The agreement, which came after a three-year stand-off over North Korea's nuclear ambitions, was hailed as an important breakthrough.
However, in a statement broadcast on North Korean radio early on Tuesday morning local time, Pyongyang reiterated its "right to peaceful nuclear activities".
It said the US "should not even dream" it would dismantle its nuclear arsenal until Washington had provided it with a light-water nuclear reactor.
Soon afterwards, Vice-Foreign Minister Kim Kye-gwan told reporters that his nation was not prepared to make the first move.
"They are telling us to give up everything, but there will be no such thing as giving it up first," he said.
The nuclear dispute began in late 2002, when the US accused North Korea of having a uranium-based nuclear arms programme, in violation of international agreements.
Oct 2002: US says North Korea is enriching uranium in violation of agreements
Dec 2002: North Korea removes UN seals from Yongbyon nuclear reactor, expels inspectors
Feb 2003: IAEA refers North Korea to UN Security Council
Aug 2003: First round of six-nation talks begins in Beijing
Feb 2005: Pyongyang says it has built nuclear weapons for self-defence
Sep 2005: N Korea agrees to give up nuclear goals
Since then four rounds of nuclear talks - between Russia, China, Japan, the US, South and North Korea - have failed to break the impasse.
Even before Pyongyang's latest comments, analysts warned that there were still many hurdles ahead in finding a lasting solution to the problem of North Korea's nuclear ambitions.
But there was one positive development on Tuesday. Japan's foreign minister said that Tokyo and Pyongyang would soon resume bilateral talks, restarting negotiations that have been stalled for nearly a year.
Relations between the two nations have been strained by the issue of Japanese citizens kidnapped by North Korea in the 1970s and 80s.