WASHINGTON, March 12, 2008 (AFP) - The United States is looking for a flexible way to unblock North Korea nuclear talks without letting the Stalinist state off the hook of full disclosure, officials said Wednesday.

One option set to be discussed at talks in Geneva Thursday is a separate declaration, addressing US concerns that North Korea has been secretly exporting nuclear technology to Syria, a diplomatic source said.

That would enable fuller six-party talks to resume, the source said, putting renewed pressure on North Korea to come clean on the full extent of its publicly disclosed nuclear program.

'We'll look at any and all ideas with the understanding that at the end of the day, we need a complete and correct declaration,' chief US negotiator Christopher Hill told reporters before flying to Geneva.

'How we get that, what the pieces of paper look like, I think we should be a little flexible on the format, but with the understanding that flexibility on format doesn't mean flexibility on getting a complete and correct declaration.'

North Korea last year signed a landmark deal to abandon all its nuclear weapons in exchange for badly needed energy and economic aid, along with major security and diplomatic benefits.

But the process -- involving the United States, China, both Koreas, Russia and Japan -- has been stalled since North Korea missed an end-2007 deadline to declare all its nuclear programs and disable a plutonium plant.

'I think we have some ideas that are maybe workable but they are only workable in the context of providing a complete and correct declaration,' said Hill, the top State Department official for East Asia.

'We cannot have a situation where they don't give us a complete declaration and where we try to ignore elements that need to be there.'

North Korea has blamed Washington for the deadlock, citing a US failure to remove it from a list of state sponsors of terrorism.

US accusations of North Korean complicity in a secret Syrian atomic drive, and allegations that Pyongyang has a furtive program to produce highly enriched uranium, have complicated the process.

Last September, Israeli military jets reportedly bombed a site in Syria after Israeli and US intelligence determined the location housed a partially built nuclear reactor constructed with North Korean help.

Washington, which is under pressure from security hawks to get tough with North Korea, has already rejected an initial declaration by Pyongyang that failed to address secret nuclear technology transfers to Syria.

But the separate declaration being negotiated in Geneva between Hill and his North Korean counterpart, Kim Kye-Gwan, would allow the six-party talks to get back on track, the diplomatic source said.

The declaration would be submitted to the other countries in the process and made public, the source added, declining to be identified.

Whatever emerges cannot be a 'secret agreement' but must be transparent, Hill said, before being asked if he was feeling upbeat about his trip to Switzerland.

'I never talk about optimism or pessimism, except with the Red Sox,' the US official said, referring to his Boston baseball team.