North Korea has offered to halt its nuclear activities in return for "corresponding measures" by the US.
Talks are focused on programmes at the Yongbyon nuclear site
Pyongyang called on Washington to give up its "hostile policy" towards North Korea, and reiterated demands for security guarantees from the US.
The offer was made at six-nation talks in Beijing. Other parties are reported to have welcomed the offer.
But it was unclear whether it included North Korea's alleged uranium enrichment project programme.
In Washington, US Secretary of State Colin Powell expressed cautious optimism about the talks.
"There is a promising attitude that is emerging from those meetings and
hopefully we can move in the right direction there," he told the Senate Budget
Pyongyang's offer came as South Korea, Russia and China offered the North oil aid to try to resolve the crisis.
"The various parties welcomed the proposition by the DPRK (North
Korea) for the comprehensive stopping of their nuclear activities," Chinese foreign ministry spokesman Liu Jianchao told reporters.
Mr Liu said the proposal was still being discussed.
China, US, Russia, Japan, North and South Korea taking part
Set to last until Friday
Parties sit at hexagonal table, US next to N Korea
It was unclear whether the offer referred both to North Korea's publicly acknowledged plutonium project and the uranium project which the US accuses it of developing.
The BBC's Charles Scanlon says analysts fear the offer could be meaningless if it does not refer to both.
The US has rejected proposals by Pyongyang to freeze its plutonium programme, and the North has always denied, in public at least, the existence of a separate uranium project.
The North Korean officials said the US would be responsible if the talks failed, but he said the North would continue to work hard to close the gap.
The announcement came as South Korea, China and Russia offered energy aid to the North if it froze and then scrapped its nuclear programme.
Talks on the crisis will resume on Friday.
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But US says freeze not enough
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N Korea denies programme exists
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N Korea says subject not relevant to nuclear talks
North Korea's economic problems have led to severe energy shortages, which were exacerbated by a US-led decision to suspend shipments of fuel aid to the country.
"The energy aid requires a presumption that North Korea freezes its nuclear activity as a beginning step to dismantle all of its nuclear programs completely, irreversibly and verifiably," South Korea's Lee Soo-hyuck said.
A similar deal was struck between the US and impoverished North Korea in 1994 but collapsed after Washington said in October 2002 that Pyongyang had privately admitted to the enriched uranium programme.