Israel rejected out of hand on Friday a mooted deal between world powers and Iran, just as US secretary of state John Kerry prepared to join nuclear talks that aim to nail down an interim agreement on the decade-old standoff.
Western diplomats say that a deal at the negotiations in Geneva is far from certain, and it would in any case mark only the first step in a long process towards settling the dispute over Tehran's nuclear programme.
Nevertheless, Israeli prime minister Benjamin Netanyahu said Iran would be getting "the deal of the century" in the talks between Tehran and six powers.
"Israel utterly rejects it and what I am saying is shared by many in the region, whether or not they express that publicly," Netanyahu told reporters.
"Israel is not obliged by this agreement and Israel will do everything it needs to do to defend itself and the security of its people," he said before meeting Kerry in Jerusalem.
Israel has repeatedly warned that it might strike Iran if it did not halt the nuclear programme, accusing Tehran of seeking to build atomic weapons. Iran says its various nuclear facilities are geared only to civilian needs.
Tehran is trying to win respite at the talks from international sanctions which are strangling its economy. The United States has said world powers will consider relaxing some of the sanctions if Iran takes verifiable steps to limit its nuclear programme.
Iran and the powers are discussing a partial suspension deal covering only a limited period. It would be the first stage in a process involving many rounds of negotiations in the next few months aimed at securing a permanent agreement.
The core of that first stage would be freeing up cash frozen in foreign accounts for years, giving Iran access to funds. Netanyahu was meeting Kerry for the third time in barely 48 hours. The US secretary of state was due to fly immediately afterwards to Geneva where Iran and six world powers are holding negotiations. These are the five permanent members of the U.N.
Security Council — the United States, Russia, China, France and Britain - plus Germany.
Israel has called for the sanctions to remain in place until Iran has dismantled its entire uranium enrichment programme. "I understand that the Iranians are walking around very satisfied in Geneva - as well they should be, because they got everything and paid nothing," Netanyahu said.
Kerry, on a Middle East tour, will fly to the Swiss city at the invitation of European Union foreign policy chief Catherine Ashton in "an effort to help narrow differences" in the negotiations, a senior State Department official said.
Ashton is coordinating talks with Iran on behalf of the six.
After the first day of meetings set for Thursday and Friday, both sides said progress had been made.
US President Barack Obama said the international community could slightly ease sanctions against Iran in the early stages of negotiating a comprehensive deal.
"There is the possibility of a phased agreement in which the first phase would be us ... halting any advances on their nuclear programme ... and putting in place a way where we can provide them some very modest relief, but keeping the sanctions architecture in place," he said in an interview with NBC News.