Kerry to join Iran nuclear talks in bid to reach deal

* Possible compromise on Iran's claimed "right to enrich" uranium

* China, British, French, German foreign ministers to join talks

* 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 (Adds Chinese Foreign Minister en route and spokesman comment)

By Louis Charbonneau and Parisa Hafezi

GENEVA, Nov 23 (Reuters) - U.S. Secretary of State John Kerry will join talks on Iran's contested nuclear programme in Geneva on Saturday, as Tehran and six world powers appeared to be on the verge of an elusive breakthrough in the decade-old dispute.

The Chinese, French, British and German foreign ministers, Wang Yi, Laurent Fabius, William Hague and Guido Westerwelle, were due to take part in intense negotiations on a deal under which Iran would curb its atomic activity in exchange for some relief from economic sanctions.

The announcements came after diplomats in the Swiss city said a major sticking point in the talks, which began on Wednesday, may have been overcome.

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.

Russian Foreign Minister Sergei Lavrov arrived in Geneva on Friday evening and met with Iranian Foreign Minister Mohammad Javad Zarif and European Union foreign policy chief Catherine Ashton, a Russian spokeswoman said.

Kerry left for Geneva "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 Ashton, who is coordinating talks with Iran on behalf of the United States, Russia, China, France, Britain and Germany, Psaki said.

Later, deputy State Department spokeswoman Marie Harf said that Kerry decided to travel to Geneva "in light of the progress being made" and with "the hope that an agreement will be reached".

Echoing optimism that a deal was close, China's state-run Xinhua news agency quoted a foreign ministry spokesman as saying the talks "have reached the final moment". The country's foreign minister left Beijing for Geneva early on Saturday.

Diplomats 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 long-sought breakthrough.

Fabius expressed hope that a deal could be made. 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.