The diplomatic maneuvering is going into high gear as the United States and its allies seek to pass sanctions on Iran and come up against resistance not just from China but from some less obvious antagonists, principally Brazil, Lebanon, and Turkey. As the debate heats up, the potential for a sharply divided vote on a sanctions resolution is increasing -- an outcome unlikely to please anyone.

According to U.N. diplomats, Lebanon has made it clear that it will not be in a position to support any sanctions resolution against Iran, which has provided military and political support to the military group Hezbollah, an influential faction in the government.

In a show of defiance, Hezbollah's leader, Hassan Nasrallah, made a rare public appearance in Damascus with Iranian President Mahmoud Ahmadinejad and Syrian leader Bashar al-Assad. "Lebanon for internal reasons is unlikely to vote for a sanctions resolution," a senior diplomat said. "I suppose they would rather avoid taking a clear yes-or-no stand on this issue."

Turkey and Brazil have also been hesitant to back sanctions against Iran. In November, the two countries abstained on a vote by the board of the International Atomic Energy Agency (IAEA) censoring Iran for its secret construction of a nuclear facility in Qom. U.N. diplomats fear they will do the same on this vote, particularly if China does.

China and Russia pressured Tehran to agree to an offer to swap its enriched uranium for a foreign supply of nuclear fuel for its medical research reactor. Their appeals -- delivered during a U.N. Security Council meeting on nuclear nonproliferation -- presented Tehran with a final chance to skirt U.N. sanctions.

Russia's U.N. ambassador, Vitaly Churkin, said that while diplomatic efforts by the U.N. and the big powers "have not yielded the desired results," there "is still a horizon for negotiations. There is still an opportunity to agree to a persuasive mutually acceptable fuel exchange model for Tehran's research reactor."

Churkin said that Tehran acceptance of the deal would constitute a "credible step" towards restoring international trust in the "peaceful nature of Iran's nuclear program." It remained unclear whether an Iranian commitment to accept the fuel exchange deal would halt the West's push for sanctions. But U.N. diplomats said it would greatly strengthen Beijing and Moscow's call for continued talks.

China's deputy U.N. ambassador Liu Zhenmin underscored Beijing's desire to see the nuclear standoff resolved through diplomatic negotiations, telling the council "we believe that sanctions are not an end in themselves and in no way can they provide a fundamental solution to this issue."

But he also reaffirmed that China remains committed to supporting the "dual track strategy" -- negotiations and sanctions -- and he urged Tehran to step up cooperation with the IAEA to "remove doubts" about the suspected military nature of Iran's nuclear program, Liu said.

"We believe the key to easing the current tense situation around this issue remains the proper solution of supply of nuclear fuel for the Tehran research reactors and the relaunching of P-5 negotiations as soon as possible with Iran," Liu said.

"The door to contact and dialogue has not closed," he added. "There's still room for further diplomatic maneuver."

The United States and its European allies are confident that they can secure at least 10 votes, including from nonpermanent members Austria, Bosnia, Gabon, Mexico, Nigeria, and Uganda -- one more than the nine required for passage in the council. But the failure to secure a united front, particularly from the five veto-wielding members of the council, would send a weak signal to the Iranians, diplomats said.

U.S. and European officials are eager to have the resolution adopted before a May 4-15 review conference on the 1970 nuclear Non-Proliferation Treaty, which sets the basic rules governing the use of nuclear weapons. One senior diplomat directly involved in the nuclear negotiations said that open-ended debate on Iran could "contaminate the atmosphere" there.

The official said that Obama's meeting with the Dalai Lama was "not very conducive" to the talks, but that Beijing may ultimately come around to sanctions: "This is not the last word out of Beijing. We have a good chance that the Chinese position might evolve."

Deputy Secretary of State James Steinberg held talks in Beijing with senior Chinese officials this week, but a Chinese foreign ministry spokesman reiterated after his departure that China believed that diplomacy had not been exhausted. State Department spokesman P.J. Crowley said Steinberg raised concerns about Iran's nuclear program.

Turkey's Prime Minister Recep Tayyip Erdogan has sought to position his government as a key power broker in the Middle East, mediating disputes between Israel and Syria and looking for a similar role in Iran. But Turkish diplomats are concerned that a vote for sanctions would jeopardize its ability to play the role of an honest broker. "They have really raised their level of diplomatic engagement quite dramatically over the last years, and the Erdogan government feels that it has a privileged relation to Tehran," said a senior ambassador involved in the talks.

Brazilian President Luiz Inacio Lula da Silva is seeking to position his government as a leader of the developing world, which often sees sanctions as a Western tool of pressure against poor countries. In New York, Brazilian diplomats have voiced concern to their counterparts that the resolution might deprive Iran of its right to possess nuclear power, according to a council diplomat.

On Wednesday, while Secretary of State Hillary Clinton was in Brazil, Lula da Silva urged the United States and its allies to continue to pursue negotiations with Iran. "It is not prudent to push Iran against a wall," Lula da Silva told reporters in Brasilia. "The prudent thing is to establish negotiations."

"I want for Iran the same thing I want for Brazil: to use the development of nuclear energy for peaceful purposes," said Lula da Silva, who will visit Iran in May. "If Iran agrees with that, Iran will have the support of Brazil."

In recent days, the United States, Britain, France, and Germany reached agreement on a set of sanctions targeting Iran's banking, insurance, and shipping sectors that they hope to include in a U.N. sanctions resolution. They are awaiting a response from Russia and China.

Churkin told Turtle Bay on Thursday that he has not received a formal response to the proposals from Moscow, and that he has no authority to begin substantive negotiations on sanctions.

But on Tuesday he voiced his country's growing weariness over nuclear negotiations with Iran, saying that Tehran has failed to provide "the appropriate responses that we expected" to peace offerings from the big powers. "Unfortunately, Tehran turned this around to the point where there was no longer any possibility for dialogue. We regret that."

"We are concerned by what we see," Churkin said. "We are concerned by the concerns expressed by IAEA. We are guided by the IAEA; we respect the IAEA, so when they are not satisfied with what they see in their cooperation we are obviously also very concerned and this adds up to things which raise worries about the nature of their nuclear program."