World leaders starting flooding into Copenhagen on Thursday, even as a Danish official acknowledged that hope was running out for a comprehensive climate deal because the negotiations between rich and poor countries were deadlocked.

The official said the Danish hosts of the U.N. conference had not given up though it appeared unlikely that their ambitious plan for the conference would be fulfilled.

"As it looks now, we will not get the deal that we had hoped for," said the official, who is not authorized to speak publicly about the talks and asked not to be named.

Denmark started the two-week U.N. conference — the largest and most important meeting on climate change in history — hoping to crafting a comprehensive framework to curb the greenhouse gas emissions that cause global warming and provide funds for poor countries to deal with climate change.

SLIDESHOW: Copenhagen Clashes.

They wanted to get agreement on all the main elements so that a treaty could be signed next year.

But so far the talks have been marked by sharp disagreements between China and the United States — the world's top carbon polluters — and a yawning chasm between rich and poor nations over what should be done.

Still unresolved are the questions of emissions targets for industrial countries, billions of dollars a year in funding for poor countries to contend with global warming, and verifying the actions of emerging powers like China and India to ensure that promises to reduce greenhouse gas emissions are kept.

European officials have called for a breakthrough in the final stretch of the conference, which is set to end Friday.

"We are in a crisis of the negotiation. We have to overcome the blockage in the discussion and negotiations," German Environment Minister Norbert Roettgen said. "We still have time: 36 or 48 hours. We need the political will to overcome this."

British Prime Minister Gordon Brown said he believed the conference would still reach an agreement.

"We can, by working together over the next 48 hours, reach agreement that will help the planet move forward for generations to come," he said.

Leaders arriving Thursday included Iran's Mahmoud Ahmadinejad, Germany's Angela Merkel, France's Nicolas Sarkozy, among many others.