Secretary of State John Kerry warned Iran it still had to prove its nuclear ambitions were peaceful as the latest round of atomic talks with world powers was due to start in Vienna and a July 20 deadline for an agreement loomed.
In Tehran, Iranian President Hassan Rouhani kept up his bullish public stance on the negotiations, saying in a speech Western sanctions imposed to curb Iran's nuclear programme were already crumbling.
The stakes are high in the Vienna talks, which will resume on Wednesday and seek a resolution to a more-than-decade-long standoff with Iran that has raised fears of a new Middle East war and a regional nuclear arms race.
Washington and some of its allies suspect Iran's programme is designed to produce nuclear weapons - a charge denied by Iran which says it is only interested in generating electricity and other peaceful projects.
Kerry's comments, published late on Monday in an op-ed article on the Washington Post website, came soon after diplomats said U.S. Deputy Secretary of State Bill Burns was expected to attend the Vienna talks.
The presence of Burns, who led secret negotiations between Iran and the United States that helped yield a November interim nuclear agreement, would open up the possibility of bilateral talks between the two long-time antagonists.
In his op-ed, Kerry chided Iran by saying its "public optimism about the potential outcome of these negotiations has not been matched, to date, by the positions they have articulated behind closed doors."
"These gaps aren't caused by excessive demands on our part," Kerry wrote, adding that the six powers had "showed flexibility to the extent possible."
The top U.S. diplomat said the July 20 deadline for a deal was "fast approaching".
Tehran, he suggested, can either seize the opportunity to do what is needed to ease concerns about its nuclear program or "they can squander a historic opportunity to end Iran's economic and diplomatic isolation and improve the lives of their people.
"There remains a discrepancy, however, between Iran's professed intent with respect to its nuclear program and the actual content of that program to date," he said.
PRESSURE ON IRAN?
In Tehran, Rouhani vowed to continue trying to get sanctions lifted in order to improve Iran's industrial sector.
"Today, some of the sanctions have been removed and others will be lifted. I will say this again - the cup of sanctions has been broken and no one will be able to return to the situation that was before," he added.
He told Iranians not to worry about the nuclear talks, saying Tehran's negotiating team was very capable.
"We know it is difficult, but we will continue down this path until the end," he said.
One of Burns's goals this week may be to press the Iranian delegation to change its negotiating stance. Western officials close to the talks say Tehran has yet to agree to sufficient curbs on the scale of its uranium enrichment program, which is among the key sticking points in the negotiations.
Burns has met the Iranians twice in the last month, first in Geneva for bilateral talks, in an apparent effort to break a logjam between Tehran and major powers on the nuclear issue, and then in Vienna, where the wider nuclear talks take place.
During the latter talks, he broached the possibility of U.S. and Iranian cooperation to try to stabilize Iraq against an onslaught by Sunni militants.
The United States along with Britain, China, France, Russia and Germany, a group collectively known as the P5+1, and Iran reached an interim pact on Nov. 24 under which Iran won some relief from economic sanctions in return for reining in some of its nuclear activities.
Their target is to reach a comprehensive nuclear agreement by July 20, but outside analysts and diplomats are deeply sceptical they can achieve this.
The new round of talks will continue until at least July 15.
For the six powers, the overarching goal is to extend the time Iran would need to assemble an atomic bomb, if it chose to do so. To achieve this, they want Iran to cut the number of centrifuges in operation. Centrifuges are machines that purify uranium for use as fuel in atomic power plants or, if purified to a high level, weapons.
Both sides have said their goal is to have a deal by July 20 and avoid a difficult extension of the interim accord that expires then. Privately, Western diplomats say they would be willing to consider extending the interim deal and continuing talks beyond July 20 only if an agreement was clearly in sight.
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