Iran and the US were racing against the clock Sunday to close in on a nuclear deal with US Secretary of State John Kerry saying it was "time to get it done" after 18 months of intense negotiations.
"If (Iran's nuclear programme is) peaceful, let's get it done. And my hope is that in the next days, that will be possible," Kerry, in Egypt but due to join the talks in the Swiss city of Lausanne later Sunday, said.
Washington wants Iran to scale down dramatically its nuclear programme in order to extend to at least a year the "breakout" time that Iran would in theory need to produce a bomb's worth of fissile material.
The Islamic republic, reeling from the pressure of sanctions, denies wanting nuclear weapons, saying its nuclear programme is for purely peaceful purposes. It wants to expand its activities.
The target is for Iran and six world powers -- the US, Russia, China, Britain, France and Germany -- to agree the outlines of a deal by March 31 and to fine-tune the details by July 1.
Added time pressure comes from the fact that Iran marks its new year on March 21, after which the country effectively closes down for 10 days. US Republicans are also teeing up legislation that could kill the whole process.
Iranian Supreme leader Ayatollah Ali Khamenei is due to make a closely watched speech Saturday.
"We believe very much that there's not anything that's going to change in April or May or June that suggests that at that time a decision you can't make now will be made then," Kerry told CBS television.
The nuclear standoff has lasted more than a decade, but the 2013 election of President Hassan Rouhani resulted in a minor thaw and the past 18 months have seen an unprecedented diplomatic effort.
Under a landmark November 2013 interim deal, Tehran stopped expanding its activities in return for minor sanctions relief. Since then the parties have been pushing for a lasting accord.
But to the alarm of Israel, US Republicans and Washington's Gulf allies, the US looks to have abandoned insisting that Iran dismantles all nuclear activities, tolerating instead a small programme under tight controls.
In theory, this still leaves Iran with the possibility to get the bomb, critics say, and last week 47 Republicans took the explosive step of writing an open letter to Iran's leaders.
They warned that any nuclear deal could be modified by Congress or revoked "with the stroke of a pen" by whomever succeeds President Barack Obama, a Democrat.
This followed a barnstorming address to US lawmakers on a Republican invitation by Israeli Prime Minister Benjamin Netanyahu -- who faces a battle to be re-elected Tuesday -- when he warned against a deal.
The letter provoked a storm in Washington with Obama saying he was "embarrassed" for the signatories, while Washington's allies in its talks with Iran were also unimpressed.
"The negotiations are difficult enough, so we didn't actually need further irritations," German Foreign Minister Frank Walter Steinmeier said.
And Iranian Foreign Minister Mohammad Javad Zarif, who arrived in Lausanne on Sunday morning, joining US negotiators, said it "told us that we cannot trust the United States."
Kerry on Saturday sought to allay concerns about a weak agreement, saying the aim was "not just to get any deal, it is to get the right deal".
Some progress has been made towards a final deal but the two sides remain far apart on several key issues.
These include the future size of Iran's uranium enrichment capacities -- which can make nuclear fuel but also the core of a bomb -- the pace at which sanctions would be lifted and the accord's duration.
"We've really made progress, particularly in the last weeks, but there are still some very difficult issues," a European diplomat involved in the talks said.
Two deadlines, in July and November, passed without an agreement but in view of the controversy in Washington -- and pressure in Iran on Rouhani to deliver -- extending yet again will be very tough.
"There is no time for additional extensions," Kelsey Davenport, an analyst at the Arms Control Association, told AFP.