U.S. Secretary of State John Kerry says "some important gaps" remain in nuclear talks with Iran, after his Iranian counterpart had indicated a preliminary agreement could be reached by Friday evening.
Kerry spoke shortly after arriving in Geneva to join the talks, interrupting a trip to the Middle East. Diplomatic sources say Russian Foreign Minister Sergei Lavrov will join his Western counterparts in Geneva Saturday to try to secure a deal over Iran's disputed nuclear program.
Lavrov told reporters in Moscow on Friday that Russia favors a solution that recognizes Iran's right to have a peaceful nuclear program and enrich uranium under the watch of the International Atomic Energy Agency.
Major world powers are concerned that Iran is trying to develop nuclear weapons. Iran says its nuclear program is for peaceful purposes.
Speaking Friday, Kerry said diplomats are working to see if they can "narrow some differences" with Iran over the issue.
"I don't think anybody should mistake that there are some important gaps that have to be closed."
Earlier, Kerry met in Tel Aviv with Israeli Prime Minister Benjamin Netanyahu, who wants Iran's nuclear program completely dismantled.
Netanyahu told reporters ahead of his meeting with Kerry that Iran "got everything and paid nothing" because it is not reducing in any way its nuclear enrichment capability.
"Iran got the deal of the century and the international community got a bad deal. This is a very bad deal and Israel utterly rejects it."
Progress was reported this week in efforts to convince Iran to scale back its nuclear program in exchange for sanctions relief.
Iranian Foreign Minister Mohammed Javad Zarif told CNN on Thursday he believes an agreement can be reached before the close of negotiations Friday.
In Washington, President Barack Obama said the deal being discussed would offer "modest relief" from the sanctions, but that most would stay in place.
"We can provide them some very modest relief, but keeping the sanctions architecture in place, keeping the core sanctions in place, so that if it turned out during the course of the six months when we're trying to resolve some these bigger issues that they're backing out of deal or they're not following through on it, or they're not willing to go forward and finish the job of giving us assurances that they're not developing a nuclear weapon, we can crank that dial back up."
It is unclear what Iran is willing to concede. Foreign Minister Zarif said Tehran is not willing to suspend its uranium enrichment program entirely, but would consider scaling it back.
This is the second meeting of the so-called P5+1 countries since Iranian President Hasan Rouhani took power in August, on promises of reaching a nuclear deal with the West.