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COPENHAGEN - Repeatedly citing the "incredibly complex" negotiations stalling a long-sought climate change deal here, senior Obama officials said hours before the president was to fly to Copenhagen, they could not predict a break-through but said they hoped the U.S. "would have an impact" on last-ditch efforts to salvage some form of compromise.

It what sounded like a concerted effort to play down expectations, two senior Obama officials said the U.N. Climate Change Conference would have to wrestle with global warming issues long after all 130 heads of state depart the Danish capital. They appeared resigned, on the eve of the talks' conclusion, to accepting vague pledges for cuts in greenhouse gas emissions rather than the binding commitment sought when talks began here two weeks ago.

"It's a challenging set of negotiations," one top official said. "This is an on-going process. This is the 15th year of this process."

White House officials said they hoped the final result would be a so-called operation agreement, weaker than a binding set of greenhouse gas limits with measurable target dates but stronger than a political commitment with no underlying action plan.

"It's in the middle," an official said.

But there was no guarantee of this outcome due to differences between the U.S. and China over pollution limits and the ability of outside nations to verify compliance - an issue covered by the umbrella term "transparency."

While Obama officials said differences on transparency had narrowed Thursday, no final deal appeared in the offing.

The officials credited an earlier offer of U.S. help in building a $100 billion annual fund starting in 2020, to help developing nations cope with the cost of pollution and deforestation limits. However, the U.S. will offer no financing unless China buckles under.

"That (U.S. financing) is contingent on a strong overall agreement that includes Chinese transparency," one senior official said, underlining a key demand laid on the table by Secretary of State Hillary Clinton when she first announced the U.S. intentions at the opening of talks on Thursday.

That wasn't the only issue bedeviling negotiators here.

The biggest, the Obama officials said, was a raging dispute between developed nations, China and developing nations about extending pollution reductions contained in the Kyoto climate change treaty. The U.S. has long since abandoned Kyoto as a guidepost for pollution control, but Canada, Japan, Russia, Australia and the European Union want it continued in some concrete form, while China and developing nations want out from under it.

"There's a lot of energy around that issue," a senior official said. "It's the issue over which it could all fail."

Asked why Obama would still make the long trek here from Washington, another top official said it was to exert "American leadership."

"We will try to have an impact here," the official said. "we don't know what the outcome of these talks will be."