Tehran and Washington began a second day of talks in Geneva on Tuesday on Iran's contested nuclear program, amid a fresh diplomatic drive in the face of a July deadline for a deal.
Washington warned of "tough choices" in the crunch negotiations, as fellow world powers braced to meet with the Islamic republic to try to build momentum.
Senior officials from Iran and the United States sat down behind closed doors in Geneva's upscale Hotel President Wilson, which was sealed off to the media. The talks were expected to last all day.
The meeting began Monday with a five-hour session, the first time since the 1979 Islamic Revolution that US and Iranian negotiators have held direct, official nuclear talks.
The two sides have met informally before, notably in a secret session last year in Oman which helped coax Tehran back to the negotiating table.
They have also sat down within the P5+1 process that involves world powers talking to Iran.
The P5+1, made up of the five permanent UN Security Council members Britain, China, France, Russia and the United States, plus Germany, secured an interim deal with Iran in November after marathon talks in Geneva.
The deadline for a final accord was July 20, but several players including Iran have already said a six-month extension may be needed.
Washington and the other P5+1 states are seeking solid commitments that will ensure Iran's stated desire for a peaceful atomic energy program is not a covert attempt to build a nuclear bomb.
The ongoing talks seek to turn an interim deal reached in November into a permanent agreement. Under the interim deal, Iran committed to limit its uranium enrichment to five percent and is gradually winning access to $4.2 billion of its oil revenues frozen abroad and some other sanctions relief.
They are then due to head to Rome to meet with Russian officials, then hold talks in Tehran with Germany on Sunday.
"Bilateral discussions offer a much more effective platform for conducting real bargaining than the cumbersome committee-type discussions in the P5+1 framework," said Ali Vaez, senior Iran analyst at the International Crisis Group.
"The two major sticking points are Iran's future enrichment capacity and sanctions relief," he said.
The goal of the bilateral talks is to prepare a June 16-20 meeting between Iran and the P5+1 in Vienna, where they aim to set down details of the final deal. The last round in Vienna in May yielded little.
'Fate of the world' at hand
Intelligence Minister Yuval Steinitz stressed Monday at the Herzliya conference that the stakes are high in the news round of talks.
"Any international agreement that leaves Iran on the threshold of nuclear capability is worse than no agreement at all," he said. "What is now at hand is not just the fate of Israel in the Middle East but the fate of the world."
Brigadier General Itai Brun, who heads the Israeli military's research division, said Iran now appeared to be talking "in earnest" about a final deal thanks to international pressure, adding that he expected an accord this year.
Iran's negotiators, meanwhile, must answer to domestic hardliners who say the country's red lines must not be crossed.
Former nuclear negotiator turned speaker of parliament, the tough-talking Ali Larijani, said Tuesday it was crucial to protect "the rights of Iranians" as well as the "scientific achievements of the peaceful activities" of the Islamic republic's researchers.
After Monday's first day of talks, Washington said more effort was needed.
"We think we've made progress during some rounds, but as we said coming out of the last one, we hadn't seen enough made. We hadn't seen enough realism, quite frankly, on the table," said deputy State Department spokeswoman Marie Harf.
"People need to make tough choices, but we are very focused on that July 20th time," she said.
Iran's deputy foreign minister and nuclear pointman Abbas Araqchi said Monday's dialogue "took place in a positive climate and was constructive," according to Iran's ISNA news agency, and may be extended for another six months.