COPENHAGEN, Denmark - The success of the U.N. climate conference hung in the balance yesterday as China and the United States deadlocked over whether Beijing will allow the world to check its books and verify promised cuts in greenhouse-gas emissions.

World leaders crowded into a vast hall for the formal opening of the largest summit ever on climate change, but attention was on the leaders of the two largest polluters - President Obama and Premier Wen Jiabao - who plan to arrive for the final days of talks on a framework to control heat-trapping gases.

Negotiators who have been working for 10 days floated new draft documents on lesser issues. But they left open the vexing 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 to ensure they keep their promises.

"In these very hours, we are balancing between success and failure," said conference president Connie Hedegaard of Denmark. Success is possible, she said, "but I must also warn you: We can fail - probably without anyone really wanting it so, but because we spent too much time on posturing, on repeating positions, on formalities."

The rest of the 115 leaders were expected to arrive before tomorrow's summit finale to sign a political outline of a global-warming treaty that would set limits on carbon dioxide pollution by the United States, China, and India, as well as extending emissions targets for the 37 countries regulated under the 1997 Kyoto Protocol.

Secretary of State Hillary Rodham Clinton was expected to arrive today to hold a round of private talks before Obama's arrival tomorrow.

Yesterday, delegates were nearing a deal to protect tropical forests, although several substantive issues remained unresolved, including targets for reducing deforestation and money to pay for conservation plans and how that money would be raised, according to the latest draft.

The program would be financed either by richer nations' taxpayers or by a carbon-trading mechanism - a system in which each country would have an emissions ceiling, and those who undershoot it can sell their remainder to over-polluters.

Meanwhile, U.S. and Chinese diplomats - Todd Stern and Xie Zhenhua - held another series of private meetings, but neither side indicated any break in the stalemate on the verification issue.

An Asian diplomat who speaks often with both sides said that the two countries had made verification a redline issue and that he feared it could cause a deal to collapse. Neither side has made any serious concessions, the diplomat said.

China promised last month to slow its carbon emissions but stressed the move would be voluntary without international assistance or financing, reflecting its reluctance to commit to internationally verifiable standards.

Washington welcomed Beijing's pledge to nearly halve the ratio of pollution to economic output in the next decade but said China should put that target in an international agreement and open it to fact-checking.

Yu Qungtain, Xie's deputy at the talks, rebuffed any verification demand that goes beyond previous agreements. "We cannot agree," he said.

The Asian diplomat said China was concerned that verification could lead to penalties for failing to meet its commitments. But he also said the United States was pushing for binding commitments from developing countries that they cannot accept.

David Doniger of the Natural Resources Defense Council, a former negotiator, saw a possible solution in a trade-off in which China would agree to have its figures reviewed in exchange for a firm U.S. offer on financing for developing countries to help them deal with rising seas, drought, and other results of global warming. The United States has said it cannot put a figure on the table until Congress legislates a climate and energy package, expected in the first half of 2010.

China is grouped with developing nations, but the United States does not consider China to be in need of climate-change aid.