* Possible compromise on Iran's claimed "right to enrich" uranium
* Russia's Lavrov in Geneva to join talks; Kerry to come too
* Iranian chief negotiator sees "considerable progress"
* Fate of Iran's Arak heavy-water reactor remains at issue
* Israel renews campaign of criticising offer to Iran
U.S. Secretary of State John Kerry will travel to Geneva to join talks on Iran's nuclear programme, the State Department said on Friday, as Tehran and six world powers appeared closer to clinching an elusive breakthrough.
Washington's announcement came after diplomats in the Swiss city said a major sticking point in negotiations on an agreement under which Tehran would curb its contested atomic activities may have been overcome.
Kerry would leave for Geneva later on Friday "with the goal of continuing to help narrow the differences and move closer to an agreement," State Department spokeswoman Jen Psaki said.
The decision was taken after consulting with European Union foreign policy chief Catherine Ashton, who is coordinating talks with Iran on behalf of the six powers, Psaki added in a statement.
Diplomats earlier said a compromise over Iran's insistence that its "right" to enrich uranium be internationally recognised has been proposed, possibly opening the way to a breakthrough in intensive negotiations that began on Wednesday.
Russian Foreign Minister Sergei Lavrov arrived in Geneva on Friday evening and met with Iranian Foreign Minister Mohammad Javad Zarif and with Ashton, a Russian spokeswoman said.
French Foreign Minister Laurent Fabius expressed hope that a deal could be made, telling reporters in Paris that he was in contact with the negotiators in Geneva. France has taken a harder line than other Western powers and repeatedly urged the six-power group not to make too many compromises with Tehran.
"As long as there is no agreement, there is no agreement. You know our position ... it's a position based on firmness, but at the same time a position of hope that we can reach a deal," Fabius said.
The United States and other Western powers say there is no such thing as a right to enrich - a process that can yield both electricity and nuclear bombs - but Iran views it as a matter of national sovereignty and crucial to any deal that would resolve a decade-old standoff over its nuclear intentions.
The Islamic Republic also wants relief from economic sanctions in return for any nuclear concessions that could allay the West's suspicions that its nuclear fuel-making programme has military rather than its stated civilian goals.
Foreign ministers from the six nations negotiating with Iran - the United States, Russia, China, France, Britain and Germany - waded into the previous talks on Nov. 7-9 and came close to winning concessions from Iran which they count on to reduce the risk of Iran achieving a nuclear weapons capability.
POLITICALLY CHARGED DETAILS
In the days running up to the talks, policymakers from the six powers said an interim accord on confidence-building steps could be within reach to start a cautious process of detente with Iran and banish the spectre of a wider Middle East war.
Under discussion is Iranian suspension of some sensitive nuclear activities, above all medium-level uranium enrichment, in exchange for sanctions relief. That could involve releasing some Iranian funds frozen in foreign bank accounts and allowing trade in precious metals, petrochemicals and aircraft parts.
The United States might also agree to relax pressure on other countries not to buy Iranian oil. Tehran has made clear it wants more significant gestures diluting the sanctions blocking its oil exports and use of the international banking system.
Diplomacy on Tehran's nuclear aspirations has revived remarkably since the election of Hassan Rouhani, a relative moderate, as president in June on promises of winning sanctions relief and diminishing Iran's international isolation.
But the sides have struggled to wrap up a deal, bogged down in politically vexed details and hampered by mutual mistrust.
Diplomats said new, compromise language of a deal being discussed did not explicitly recognise a right to produce nuclear fuel by any country. "If you speak about the right to a peaceful nuclear programme that's open to interpretation," a diplomat told Reuters without elaborating.
No other details were available, but Zarif, Tehran's chief negotiator, said earlier in the day that significant headway had been made even though three or four "differences" remained.
The fate of Iran's Arak heavy-water reactor project - a potential source of an alternative bomb material, plutonium - and the extent of sanctions relief were among the other stumbling blocks, diplomats said.
The OPEC producer rejects suspicions it is covertly trying to develop the means to produce nuclear weapons, saying it is stockpiling nuclear material for future atomic power plants.
SENATE SANCTIONS PUSH
A senior European diplomat told reporters earlier that foreign ministers of the six states would come to Geneva only if there was a deal to sign. "We have made progress, including core issues," the diplomat said.
Zarif and Ashton met throughout the day on Friday to try to narrow the remaining gaps.
Israel continued its public campaign of criticising the offer of sanctions rollbacks for Iran, voicing its conviction that all it would achieve would be more time for Iran to master nuclear technology and amass potential bomb fuel.
"We think it's not a useful agreement, perhaps even damaging," Deputy Foreign Minister Ze'ev Elkin told Israel Radio.
For the powers, an interim deal would mandate a halt to Iran's enrichment of uranium to a purity of 20 percent - a major technical step towards the bomb threshold, more sweeping U.N. nuclear inspections in Iran and an Arak reactor shutdown.
The United States has only limited flexibility during the talks, however, because of scepticism in the U.S. Congress about the benefits of cutting any deal with Tehran.
U.S. Senate Majority Leader Harry Reid said on Thursday he was committed to pursuing a tougher Iran sanctions bill when the Senate returns from a recess early next month - even though President Barack Obama has warned that could derail diplomacy in Geneva.
The White House said on Friday it hoped a deal can be reached in Geneva. If a preliminary agreement is reached for a six-month suspension of some of Iran's most sensitive nuclear activity, the six powers and Tehran will use that time to hammer out a broader and longer-term accord.