Secretary of State John Kerry will travel to Geneva Friday to join international talks on Iran's disputed nuclear program, fuelling hopes a historic deal may be in sight.
Tehran and world powers ended a first day of talks Thursday with Iranian Foreign Minister Mohammad Javad Zarif saying a deal could be reached "before we close these negotiations."
Negotiators from Iran and six global powers are meeting for two days in Geneva to broker a deal that could see Tehran freeze its nuclear efforts in exchange for some relief from the sanctions that have battered its economy.
Western powers suspect Iran's uranium enrichment may be aimed at developing nuclear weapons, a claim Tehran denies.
Kerry will go to the Swiss city "in an effort to help narrow differences in negotiations" and at the invitation of EU foreign policy chief Catherine Ashton, a senior State Department official said.
Upending an 11-day tour mostly of the Middle East, Kerry was due to arrive in Geneva later Friday for the talks which had dragged for years until new Iranian President Hassan Rouhani came to power in August.
Kerry, who was in Amman Thursday, will first fly to Tel Aviv to meet with Israeli Prime Minister Benjamin Netanyahu, and his surprise decision to go to Geneva is sure to infuriate key US ally Israel. Iranian leaders in the past have denied the Holocaust and threatened to destroy the Jewish state.
Any deal with the Islamic republic would be "a mistake of historic proportions," Netanyahu warned.
Iran's Zarif was due to meet early Friday with EU diplomatic chief Ashton, who is chairing the talks on behalf of the P5+1 group of world powers -- permanent UN Security Council members Britain, China, France, Russia and the United States, plus Germany.
In their second meeting in Geneva in less than a month, Iranian negotiators sat down for a series of talks that Western officials described as "substantive" and "productive."
"There is a window of opportunity now that has been created by the Iranian people... and that opportunity needs to be seized," Zarif said.
After the talks, a spokesman for Ashton said "we are making progress" but that it was too early to speak of an end game.
"I can't give you any sort of final verdict yet," said the spokesman, Michael Mann. "The ball is in their court."
In another possible indication the talks were making headway, Zarif cancelled a planned trip to Rome to stay on in Geneva.
Iran's lead negotiator, Deputy Foreign Minister Abbas Araqchi, said Zarif was staying because talks "have entered a complicated, difficult and intensive" phase.
The meeting is the second since Rouhani took office in August pledging to resolve the nuclear dispute and lift sanctions by engaging with world powers.
Iran is anxious for relief from crippling economic sanctions that have cut oil revenues by more than half, caused the value of the rial to plunge and pushed inflation above 40 percent.
The West is also keen to seize a rare opportunity to build bridges with Iran after decades of hostility, opening the door to engaging with Tehran on other issues like the conflict in Syria, where Iran has backed President Bashar al-Assad against insurgents.
US President Barack Obama said in an interview with NBC News that the agreement being fleshed out would keep the bulk of sanctions on the Islamic Republic in place, and any relief could be reversed.
"We don't have to trust them. What we have to do is to make sure that there is a good deal in place from the perspective of us verifying what they're doing," Obama said.
"There is the possibility of a phased agreement in which the first phase would be us halting any advances on their nuclear program, rolling some potential back, and putting in place... some very modest relief, but keeping the sanctions architecture in place."
Last month's talks in Geneva -- held in English for the first time -- saw Iran reportedly outline a two-stage process that would resolve the dispute within a year.
Washington and Tehran have not had diplomatic relations for more than three decades, following the 1979 Islamic revolution and the subsequent 444-day siege of the US embassy in the Iranian capital.
Encouraged by the new approach by Rouhani, who is seen as a relative moderate, Kerry met with Zarif at the United Nations in September. Obama also made a historic first phone call to Rouhani.
The six international powers have been pushing Iran to freeze its enrichment efforts, reduce stockpiles and lower its capacity to produce nuclear material.