The U.N. Security Council is expected to vote on a resolution to impose additional sanctions over Iran's nuclear program. Meanwhile, Tehran makes diplomatic efforts to head off new penalties.

With a critical U.N. Security Council vote looming, Western diplomats appealed to Brazilian officials Tuesday to drop their opposition to a new battery of international sanctions against Iran.

Iranian officials, meanwhile, continued pursuing their own diplomatic efforts to head off new penalties. The Security Council was expected to vote Wednesday on a resolution to impose a new round of sanctions, the fourth since 2006.

At least 12 of the United Nations Security Council's 15 members are expected to vote to approve the sanctions, which are aimed at pressuring Iran to limit its nuclear program. But in the interest of displaying international unanimity, the resolution's supporters would prefer that countries skeptical of new sanctions abstain rather than vote "no."

Avoiding "no" votes "would make a big difference symbolically," said an official of a country that favors the new sanctions, speaking on condition of anonymity in keeping with diplomatic protocol.

Iran argues that it's developing a nuclear program only for peaceful purposes; the United States and many other countries suspect it is seeking to develop atomic weapons.

Brazil, a Security Council member, has criticized the proposed sanctions, which seek to punish Iran's nuclear industry, limit nonnuclear arms sales and authorize ship inspections to prevent imports of nuclear-related equipment. The resolution adds 41 organizations and a scientist to a blacklist.

U.S. officials met Monday with Brazil's deputy foreign minister to discuss the sanctions. An abstention by Brazil could make it harder for Turkey and Lebanon, which also have criticized the proposed resolution, to vote "no."

The Security Council's major powers, including Russia, China and the U.S., are expected to vote to approve the sanctions. The proposed measure was weakened by U.S. officials and their allies in a bid to win assent from Russia and China, which generally have resisted sanctions proposals against Iran.

The three previous rounds of U.N. Security Council sanctions all have been adopted unanimously.

U.S. officials contend the new sanctions will demonstrate decisive international disapproval of Iran's nuclear program. But none of the past sanctions efforts have brought Iran to the negotiating table, and critics doubt the latest resolution will be different.

Supporters of U.N. actions believe it will provide legitimacy for subsequent measures expected to be imposed in coming weeks by the European Union and various states, including the U.S., Australia and Canada.

Brazil and Turkey announced last month that they had reached agreement with Iran aimed at averting the latest round of sanctions. But the deal, which would have shipped some of Iran's enriched uranium out of the country, was dismissed by most world powers as insufficient to address concerns that Iran was progressing in its efforts to gain nuclear weapons know-how.

As part of a campaign to stave off sanctions, Iranian President Mahmoud Ahmadinejad visited Turkey on Tuesday and will travel to China on Wednesday.

Ahmadinejad warned Tuesday in a meeting with Turkish and Russian leaders that Iran would not be willing to negotiate once the U.N. had approved more sanctions.

"If the U.S. and its allies think they could hold the stick of sanctions, and then sit and negotiate with us, they are seriously mistaken," he told reporters, according to Iran's state-owned Press TV.