WASHINGTON  - The United States called Wednesday for a united front with China against Iran's nuclear program as it debates whether to slap sanctions on the Asian power over oil purchases from the Islamic republic.

The United States has exempted 18 nations but not China from tough sanctions that come into effect on June 28 on countries that buy oil from Iran, which Israel and some Western officials accuse of building a nuclear weapon. Kurt Campbell, the top State Department official on East Asia, said that the United States and China were "right in the middle" of talks about Iran and did not answer a question on whether Beijing would receive an exemption. "We have underscored how important it is to have a solid, unified international consensus about how to deal with the challenges posed by Iran's nuclear program," Campbell said at the Center for a New American Security, a Washington think-tank.
But Campbell welcomed China's efforts in the so-called P5+1 - a group comprising Britain, China, France, Germany, Russia and the United States that is negotiating with Iran over its nuclear program.  "I must say we have thanked China for their support within the P5+1 and we will continue close consultations with them going forward," he said. China has defended its oil purchases, saying that they were legal and transparent and criticizing the United States for imposing sanctions unilaterally instead of working through the United Nations.
Meanwhile, Russian FM Sergei Lavrov flew into Iran on Wednesday for a brief visit to discuss upcoming international talks over Tehran's disputed nuclear programme. The trip preceded a new round of negotiations between Iran and the major powers that is to be held in Moscow next Monday and Tuesday.
In a joint news conference after meeting his Iranian counterpart, Ali Akbar Salehi, Lavrov revealed little of what they talked about. "The Iranian side is interested in coming up with solutions which would contribute to the settlement of the nuclear issue," he said, speaking through an official interpreter. He reiterated Russia's opposition to unilateral sanctions imposed by Western countries that are hurting Iran's oil export-dependent economy. Salehi said he was "optimistic" about the prospects of the Moscow negotiations, despite two unproductive rounds in Istanbul and Baghdad earlier this year.
"The direction taken by the two sides to resolve the issue is the right one," he said. "The issue is complicated and one has to have patience to make progress."
He added: "In this process, it can slow down at times, then accelerate. But we are optimistic about the final result."
Lavrov was also to meet Iran's lead negotiator, Saeed Jalili, before flying out, according to Iranian officials.
US Secretary of State Hillary Clinton said on Tuesday that the Iranians "are under tremendous pressure from the Russians and the Chinese to come to Moscow prepared to respond" to proposals by the world powers to alleviate the showdown over Tehran's nuclear activities.
She said: "The Russians have made it very clear that they expect the Iranians to advance the discussion in Moscow. Not just to come, listen and leave. We will know once it happens."
Moscow is the most sympathetic to Tehran among the six powers negotiating with it in the talks, although it has sided with the West in expressing fears that Iran could be pursuing the development of a nuclear weapons capability, which has raised the spectre of military strikes by the United States or Israel.
The so-called P5+1 group of nations - comprising UN Security Council permanent members Britain, China, France, Russia and the United States, plus Germany - offered a package of proposals to Iran in the last round, in Baghdad in May.
They called for Iran to halt its enrichment of uranium to 20 percent, ship out its stockpile of 20-percent uranium and halt enrichment at its fortified Fordo facility.
In return, they offered nuclear cooperation, spare parts for Iran's dilapidated passenger aircraft fleet and an easing of a EU ban on tanker insurance that hinders oil sales to Asia.
Iran's negotiators rejected the package as grossly insufficient. They countered with a list of their own issues that included many non-nuclear topics such as regional security, and the demand that the P5+1 override several UN Security Council resolutions by agreeing that Iran has a "right" to uranium enrichment.
The distance between the two sides' positions almost caused the Baghdad round to collapse, but last-minute discussions managed to eke out an agreement for the Moscow round, which will take place just two weeks before an EU embargo on Iranian oil imports is due to to be fully imposed.
In the past few days, Iranian officials have softened slightly their stance by saying the issue of 20-percent enriched uranium could still be up for negotiation - but only if the concession offered in return was of the same importance.