North Korea says it will not consider halting its nuclear programme unless UN sanctions imposed after it tested a nuclear device in October are lifted.
Christopher Hill has warned that US patience is wearing thin
The condition was part of the North's tough opening statement as six-nation talks on the issue resumed in Beijing after a one-year suspension.
The North warned it would increase its nuclear deterrent if its demands were not met.
The US said it was "running out of patience" with the North.
The Beijing talks also include China, Japan, Russia and South Korea.
Correspondents say the North often adopts a tough initial stance as a negotiating ploy.
The North's delegation is expected to meet the Americans directly later on Monday.
The statement at the opening of the talks on Monday by North Korean envoy Kim Kye-gwan was defiant in tone.
The North said it was unconcerned that other countries did not accept its newly demonstrated nuclear status.
The North says it should be treated on equal footing with the US and that talks should now be about arms reduction.
The statement repeated a demand for a nuclear reactor to generate energy.
It also said the US must lift financial sanctions it imposed more than a year ago.
North Korea walked out of the last round of talks in November last year in protest at the US restrictions - imposed on a Macau-based bank linked to alleged money-laundering by Pyongyang.
Two months earlier, in September 2005, Pyongyang had agreed to abandon its nuclear programme in return for US security guarantees and aid in a deal that was hailed as historic.
Kim Kye-gwan (right) has brought a list of demands from Pyongyang
One South Korean official said of Monday's opening statement: "The gist was that they would be able to dismantle nuclear weapons only after the United States dropped what they think is a hostile policy."
The US says it has no intention of accepting North Korea as a nuclear power.
Washington's envoy to the talks, Christopher Hill, expressed frustration at the North's defiance.
"The supply of our patience may have exceeded the international demand for that patience, and we should be a little less patient and pick up the pace and work faster," he said.
Observers say there will be relief the talks are resuming but scepticism about the chance of a major breakthrough.
UN sanctions were imposed on North Korea after it tested a missile in July and then exploded its first nuclear device in October, leading to a wave of international condemnation.
Separately on Monday Japanese officials rejected claims by North Korea that Japan had kidnapped one of its citizens.
Pyongyang's official news agency said at the weekend that a North Korea linguist went missing in 1991 and was presumed to have been abducted by the Japanese.
The BBC's Chris Hogg in Tokyo says many in Japan will see the allegation as an attempt to counter pressure from the Japanese on the abduction of its citizens by the North.
Japan believes at least 17 of its nationals were taken in the 1970s and 1980s, and probably many more.
Pyongyang has returned five and insists the others are dead.