Lavrov arrived in Geneva as diplomats were struggling to find language acceptable to Iran and its six negotiating partners.
As negotiations moved into the evening, a diplomat in Geneva for the talks said some progress was being made on a key sticking point — Iran's claim to a right to produce nuclear fuel. Iranian Foreign Minister Mohammad Javad Zarif and Catherine Ashton, the European Union's top diplomat, have met repeatedly since Wednesday trying to resolve that and other differences.
Lavrov said on Wednesday that he could meet with U.S. Secretary of State John Kerry this week, but didn't say where. That increased speculation that Kerry could join him in Geneva, along with the foreign ministers of the four other countries negotiating with Iran.
The last round of talks between Iran and the six world powers ended Nov. 10 with no deal even after Kerry, Lavrov, the foreign ministers of Britain, France and Germany and a Chinese deputy foreign minister flew in and attempted to bridge differences.
Ashton is convening the negotiations, and protocol dictates that foreign ministers come only at her invitation.
Ashton's spokesman Michael Mann said she did not issue any invitations this time. That suggested that a deal was not yet in the cards and that Lavrov was in Geneva at his own initiative.
Zarif and Ashton met briefly Friday for talks that Iran's official IRNA news agency described as "complicated and tough." It quoted Iranian Deputy Foreign Minister Abbas Araghchi in Geneva as saying that Iran's right to uranium enrichment must be part of any deal.
Iran says it is enriching only for reactor fuel, medical uses and research. But the technology can also produce nuclear warhead material.
Zarif last weekend indicated that Iran is ready to sign a deal that does not expressly state Iran's right to enrich, raising hopes that a deal could be sealed at the current Geneva round.
On Wednesday, however, Iran's supreme leader, Ayatollah Ali Khamenei, said his country would never compromise on "red lines." Since then Tehran has reverted to its original line — that the six powers must recognize this activity as Iran's right under the nuclear Nonproliferation Treaty despite strong opposition by Israel and within the U.S. Congress.
A senior Iranian negotiator said that the Iranian claim did not need to be explicitly recognized in any initial deal, despite Khamenei's comment. He did suggest, however, that language on that point remained contentious, along with other differences.
The diplomat said work was proceeding on a compromise along the lines of what the Iranian negotiator said — avoiding a direct reference to any country's right to enrich but still giving enough leeway for Iran to accept it.
Both he and the Iranian envoy demanded anonymity because they were not allowed to discuss the closed negotiations.
Sanctions relief was also an issue.
The United States and its allies have signaled they are ready to ease some sanctions in return for a first-step deal that starts to put limits on Iran's nuclear program. But they insist that the most severe penalties — on Tehran's oil exports and banking sector — will remain until the two sides reach a comprehensive agreement to minimize Iran's nuclear arms-making capacity.
Iran says it does not want such weapons and has indicated it's ready to start rolling back its program but wants greater and faster sanctions relief than that being offered.
Several U.S. senators — both Democrat and Republican — have voiced displeasure with the parameters of the potential agreement, arguing that the U.S. and its partners are offering too much for something short of a full freeze on uranium enrichment.
Senate Majority Leader Harry Reid said Thursday that he would support legislation to expand sanctions against Iran, though he said he also backs the negotiating effort. Reid said the threat of more sanctions was essential to get an acceptable deal.
Sen. Bob Corker, the Republicans' top member on the Senate Foreign Relations Committee, on Thursday proposed a bill outlining a final agreement, including an end to all Iranian enrichment activity, and seeking to restrict President Barack Obama's capacity to ease sanctions.
Associated Press writer George Jahn contributed to this report.
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