WASHINGTON (AFP) - US top diplomat John Kerry and his Iranian counterpart on Saturday launch a new high-stakes diplomatic drive to nail down an unprecedented nuclear deal as they hurtle towards a deadline just one month away.
US officials warned the weeks leading to June 30 would be intense, vowing to "keep the pressure on" to force the Iranians and everyone at the table to make the "tough decisions" needed to end a 12-year standoff and put a nuclear bomb beyond Iran's reach.
Sealing a long-elusive deal with the Islamic republic could prove President Barack Obama's lasting foreign policy achievement.
But after three decades of enmity, it may also pave the way towards better ties between Washington and the Shiite regional power, bringing Iran back into the international fold and creating fresh impetus to resolve a host of conflicts grown ever more perilous in the Middle East.
Kerry and Iranian Foreign Minister Mohammad Javad Zarif will meet once again in the Swiss city of Geneva, as the United States with its partners - Britain, China, France, Germany and Russia - seek to finalise the complex pact.
After an interim accord hammered out in the Swiss city in November 2013, the United States and Iran are grappling with the final details of the ground-breaking agreement which would see Iran curtail its nuclear ambitions in return for a lifting of a web of international sanctions.
- Time running out -
The two diplomats are due to meet on Saturday, but US officials did not rule out that the talks could stretch into a second day as the clock ticks down to the deadline.
After eight days of emotional, tough talks in Lausanne, the delegations unveiled on April 2 a framework to guide the last stretch of negotiations, with Iran agreeing to rein in and mothball large sections of its nuclear program.
But differences remain, while both the United States and Iran are under immense pressure from hardliners at home.
"We have a month left. I do think you'll see it escalate at the political level," a senior State Department official told reporters on Friday, saying Kerry wanted to meet with Zarif to discuss the "really tough sticking issues."
June is "going to be a pretty intense month both at the expert level and the secretary's level, but I think we definitely still believe we can do it."
Since the April 2 accord, technical experts have been meeting quietly behind the scenes to overcome the remaining issues. But many of the decisions now need to be made at a political level.
The US official said that the accord would have "multiple annexes" perhaps as many as three or more, which are already being drafted.
And despite rumblings from both Iranian and French officials that the talks may drag on beyond June 30, US officials insisted that was not on the table.
"We've been very clear that we are not contemplating an extension at this point. June 30th is a real date," the State Department official said.
Several sticking points still remain, including the possible military dimensions of the Iranian program and the demands by the P5+1 group for rigorous inspections of Iran's nuclear sites.
Iran also is demanding an accelerated lifting of US, EU and UN sanctions, while the P5+1 group wants a "snapback" mechanism to be put in place enabling the measures to be quickly reimposed if Tehran breaks the deal.
"Kerry will stress to Zarif the importance of granting access for inspections," a western diplomat said.
"It's one of the key legs of the stool. It's not the only leg, but if it is not included then the stool will fall over."
Yukiya Amano, the head of the UN's atomic watchdog, told AFP this week that Iran had agreed to implementing the Additional Protocol of the Nuclear Non-Proliferation Treaty (NPT) that allows for snap inspections.
"When we find inconsistency or when we have doubts, we can request access to the undeclared location for example, and this could include military sites," he said.