A fresh round of talks between Iran and world powers kicked off on Thursday with a push for Tehran to freeze its disputed nuclear program in exchange for some relief from sanctions.
Officials have said that a long-awaited deal on curbing Iran's nuclear ambitions may be finally within reach, after years of fruitless talks were given fresh momentum by the election of Iran's new President Hassan Rouhani, seen as a relative moderate.
In their second meeting in Geneva in less than a month, negotiators from the United States and five other global powers sat down with Iranian officials for two days of talks aimed at hammering out an agreement.
Chaired by Catherine Ashton - the EU diplomatic chief who heads the so-called P5+1 group of Britain, China, France, Russia, the United States and Germany - the initial talks ended after 45 minutes but were set to resume later.
"The talks are extremely complex and are now getting into a serious phase," Ashton's spokesman Michael Mann told journalists.
"It was a good opening session... We have agreed with the Iranian side that we will not go into the details, into the substance of what's being discussed in the room," he said.
"But we very much hope of course that there will be concrete progress here over the next couple of days."
In a possible indication the talks were making progress, Iranian officials said Foreign Minister Mohammad Javad Zarif had cancelled a trip to Rome.
Iran's lead negotiator in Geneva, Deputy Foreign Minister Abbas Araqchi, said in comments shown on Iranian state television that Zarif would be staying in Geneva because talks "have entered a complicated, difficult and intensive" phase.
He said Iranian officials would be meeting separately with European, Russian, Chinese and US officials Thursday before full talks resume in the early evening.
"There are serious differences still remaining but what is important is that both sides have serious will to bridge these differences," Araqchi said.
Both sides have said recent talks have been the most productive in years but admit that reaching a deal will not be easy.
Zarif nonetheless sounded an optimistic note earlier this week, saying he believed it would be "possible" to reach an agreement.
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.