World powers and Iran are expected to reach a long sought deal on Friday to try to resolve Iran's controversial nuclear programme.
US, UK, French and German foreign ministers are making unscheduled trips to Geneva to join talks between Iran and the so-called P5+1 delegates.
Details have not been released but Iran is expected to halt some enrichment activity for limited sanctions relief.
Israel's prime minister said such an agreement would be "very bad".
The West suspects Iran's uranium enrichment programme is a step towards building nuclear weapons - a charge Iran strongly denies.
Iranian Foreign Minister Mohammad Javad Zarif told CNN on Thursday that he was hopeful all sides could reach an agreement before the talks ended on Friday.
As reports of a deal emerged, US Secretary of State John Kerry, who was in Israel on a Middle East tour, changed his itinerary to fly to Geneva instead.
The BBC's Kim Ghattas, who is travelling with Mr Kerry, says his dramatic decision to change his travel plans is a clear sign that a deal with Iran may be within reach.
State department official Jen Psaki told the BBC Mr Kerry was going "to help narrow differences in negotiations" between Iran and the P5+1 (US, Russia, China, the UK, France and Germany).
UK Foreign Secretary William Hague, French Foreign Minister Laurent Fabius and German Foreign Minister Guido Westerwelle are also going to the talks.
Mr Zarif told CNN Iran would not suspend uranium enrichment completely but could "deal with the various issues on the table".
The US confirmed some sanctions relief was being offered in return for "concrete, verifiable measures".
"We can provide them some very modest relief, but keeping the sanctions architecture in place," President Barack Obama told NBC News.
"So that if it turned out during the course of the six months when we're trying to resolve some of these bigger issues that they're backing out of the deal, they're not following through on it or they're not willing to forward and finish the job of giving us assurances that they're not developing a nuclear weapon... we can crank that dial back up," he said.
But speaking ahead of meeting John Kerry on Friday morning, Israel's Prime Minister Benjamin Netanyahu said such a deal would be a serious mistake.
"I understand that the Iranians are walking around very satisfied in Geneva - as well they should be, because they got everything and paid nothing.
"Everything they wanted, they wanted relief of sanctions after years of a gruelling sanctions regime, they got that, they are paying 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, this is a very bad deal. Israel utterly rejects it and what I am saying is shared by many, many in the region, whether or not they express that in publicly.
"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."
Israel sees a nuclear armed Iran as an existential threat. Mr Netanyahu has said the international community should accept nothing short of a complete halt to Iran's entire uranium enrichment programme, the removal of all enriched uranium from its territory, the closure of its underground uranium enrichment facility at Fordo, and a halt to construction of its heavy water reactor near Arak.
Mr Zarif said the sides could sit down by Friday morning to prepare "some sort of a joint statement" that would address three elements - a common objective, an "end game... in less than a year'' and mutual confidence-building measures.
Iran's lead negotiator, Deputy Foreign Minister Abbas Araqchi, said the world powers had "clearly" accepted his country's proposed framework and were now discussing details.
However, there was no official confirmation from the P5+1.
Hopes of a long-awaited deal on curbing Iran's nuclear ambitions were given fresh momentum this year by the election of President Hassan Rouhani, seen as a relative moderate.
Since 2006 the UN Security Council has imposed a series of sanctions - including asset freezes and travel bans - on entities and people involved in Iran's nuclear programme.
Separate US and EU sanctions have targeted Iran's energy and banking sectors, crippling its oil-based economy. Iran wants the sanctions lifted.