Optimism about an imminent breakthrough in nuclear talks with Iran appeared to be misplaced after US Secretary of State John Kerry warned that significant differences remained.

Iran's top negotiator said that six major world powers had accepted framework proposals on Thursday and progress looked all the more likely as foreign ministers from Britain, France and Germany flew to Geneva to take part in the talks.

But Mr Kerry said: "Some very important issues on the table are unresolved.

"There is not an agreement at this point in time."

The talks focused on the size and output of Iran's enrichment programme, which the US and its allies fear could be used to produce nuclear weapons.

Iran insists its nuclear programme is purely for civilian energy.

Washington is apparently looking for Iran to stop any nuclear activities that could be used for military purposes.

Russian Foreign Minister Sergey Lavrov was not planning on going to Geneva and there was no word from Beijing on whether the Chinese minister planned to attend.

French Foreign Minister Laurent Fabius was first to touch down, saying: "I've come to Geneva to take part in the negotiations because the talks are difficult but important for regional and international security."

Israel meanwhile has been striking its regular jingoistic pose, with Prime Minister Benjamin Netanyahu saying he "utterly rejects" any deal and the country would do what it needs to defend itself.

Mr Netanyahu has repeatedly threatened unilateral military action against Iran if and when he thinks diplomacy has failed.

"I understand the Iranians are walking around very satisfied in Geneva, as well they should because they got everything and paid nothing," he said.

"They paid nothing because they are not reducing in any way their nuclear enrichment capability. So Iran got the deal of the century and the international community got a bad deal."

He insisted Israel was not bound by any agreement.

Mr Kerry had been visiting Tel Aviv, where he laid into Israel's policies towards Palestinians.

White House spokesman Josh Earnest said the US and Israel were in "complete agreement about the need to prevent Iran from obtaining a nuclear weapon."