DOHA, Qatar -- Rich and poor countries are still at odds over who will bear the bulk of the burden to combat global warming as U.N. climate talks go into overtime.

U.S. Special Envoy for Climate Change Todd Stern said today that America cannot accept the latest proposal to wrap up negotiations that include several contentious issues, like how financial aid to poor countries will be structured.

"Progress has been made, to be sure, in our time here in Doha," Stern said. "But I do want to underscore that the United States has serious concerns with the text."

Among the provisions he said the United States could not endorse was language assigning developed and developing nations different levels of responsibility to ratchet down emissions "and take into account the imperatives of equitable access to sustainable development, the survival of countries and protecting the integrity of Mother Earth."

Youth protesters push for a more ambitious agreement at the close of the U.N. climate talks in Doha, Qatar. Photo by Jean Chemnick.

Stern also flagged passages on intellectual property rights and multilateral trade as redline issues for the United States, Canada and Australia.

The objections brought an angry reaction from Saudi Arabia, whose negotiator accused the United States and others of a "mass destruction approach."

The battle is expected to last well into the weekend. Already, the organizers of the Conference of the Parties to the U.N. Framework Convention on Climate Change (UNFCCC) have extended the cafeteria hours and shuttle buses from hotels to the conference center through Saturday evening.

"That's not a good sign," said Saleem Huq, a senior fellow at the International Institute for Environment and Development in the United Kingdom.

The annual battle at the UNFCCC this year is more subtle but also more explosive than in years past. Countries last year agreed to start new talks toward a global climate change agreement that would for the first time see all nations -- including the United States and China -- take legally binding carbon cuts.

First, though, negotiators are bound to close a group of discussions that began five years ago. While technical and obscure, the fight matters tremendously for developing countries, which see this conference as their last, best hope of wringing promises from the wealthy to deliver the resources they need to cope with the effects of climate change.

"I think it's, frankly, kind of a trust issue," said Jennifer Haverkamp, international climate policy director for the Environmental Defense Fund.

"Needs are so great that the lack of certainty in greater commitment is very difficult for the most vulnerable countries to accept," she said.

Developed countries promised to deliver $30 billion in climate aid between 2010 and 2012. They say they have delivered and now are working on ways to fulfill another promise: mobilizing $100 billion in annual climate change assistance by 2020.

But developing countries are worried about those middle years and are pushing for a promise to maintain contributions and increase them to at least $60 billion between 2013 and 2015.

"There is no money that we can see on the table right now," said Marshall Islands Minister in Assistance to the President Tony de Brum, speaking for a group of small island nations.

"We will see how far we can go on that," said Pa Ousman Jarju of Gambia, representing a group of least developed countries.

"We want a medium-term finance gap to be filled, and what we want is a trajectory toward the $60 billion," he said. Developing countries expect the bulk of that money to come from government coffers, he added.

Fiscal cliff

The United States, on the precipice of a so-called fiscal cliff, has no intention of allowing an exact dollar figure to be inserted in the text, observers said. U.S. and E.U. negotiators also are wary of wording that promises to deliver steadily more, though several nations have pledged individually.

Most of the support has come from European countries, led this week by Britain, which promised Tuesday to provide $2.8 billion for each of 2013 and 2014. Germany, France, Sweden, Denmark and the European Commission all made specific pledges for one or two years. Canada also made an announcement about how the remainder of its fast-start financing would be spent.

The United States hasn't followed suit, but Haverkamp said that was somewhat understandable given the financial discussions the White House and Congress are engaged in at home.

"All that focus right now is sucked up with the fiscal cliff," she said, "and probably the best thing the U.S. can do right now for climate finance is get past the fiscal cliff."

Still, other countries will soon need more certainty about the contributions the world's largest economy and largest historic emitter will make to the climate finance pot, she said. America is also not the only laggard -- observers had hoped, for example, that Doha would put up a sum of money, and it has not.

"There are a fair number of rich countries in the world, one of which is hosting a COP [Conference of Parties]," Haverkamp said.

Text unveiled Friday afternoon calls for a work program next year "to contribute to the ongoing efforts to scale up mobilization of climate finance" to the $100 billion mark in 2020 -- wording that poor countries and their advocates here describe as weak.

"We are standing with those countries that are sticking their necks out against a bad deal here," said Cameron Fenton, 26, of the Canadian Youth Climate Coalition. He said even $60 billion isn't enough, noting that the governors of New York and New Jersey are calling for more than that to deal with the impacts of Superstorm Sandy.

"If we're seeing one storm cost $60 billion, that's nowhere near enough for the whole world," Fenton said.

Jarju said he is not yet ready to stage a walkout over money, saying he still has hope.

"We want to have a positive exchange. We have not reached the stage where we would walk out," he said.

Activists, though, said if it comes to that, they will stand behind the islands and poor nations.

"Finance is a make-or-break issue," said Celine Charveriat of Oxfam. "If they feel it's time to walk out, we will support them."