Experts from the International Atomic Energy Agency (IAEA) arrived in Tehran on early Wednesday to try resolve long-running differences with Iran over its controversial nuclear programme.
The eight-strong team, led by the UN agency's chief atomic inspector and deputy director Herman Nackaerts, was greeted at the airport by Iran's envoy to the IAEA, Ali Asghar Soltanieh, the ISNA news agency reported.
On Tuesday, before flying out from the agency's base in Vienna, Nackaerts had called on Iran to be "constructive".
"Throughout this process, the director general has always said that we are approaching these talks in a constructive spirit," he told reporters.
"Also this time we are approaching it in the same spirit, and we trust that Iran will work with us in the same spirit," he added.
But the IAEA's hopes of reaching a deal are not high.
IAEA head Yukiya Amano said on Friday he was "not necessarily optimistic," while a Western diplomat told AFP on Sunday "there still remain some pretty big disagreements" with Tehran.
The agency wants Iran to respond to what it calls "overall, credible" evidence of nuclear weapons research having been carried out until 2003 -- and possibly since then.
Iran vehemently denies having ever sought an atomic bomb.
An Iranian foreign ministry spokesman said on Tuesday that the government hoped for a comprehensive agreement with the IAEA.
But Ramin Mehmanparast said that would only be possible if the agency recognised Iran's "nuclear rights."
Mehmanparast appeared to play down the chances that the IAEA team might get access to Parchin, a military base near Tehran where the agency's experts suspect Iran could have carried out experiments with explosives capable of triggering a nuclear weapon.
"Parchin has no connection with Iran's nuclear activities," Mehmanparast said. Access to it could be discussed, but only in the context of a possible agreement, he added.
Wednesday's talks will be closely monitored by the so-called P5+1 group -- Britain, China, France, Russia, the United States and Germany. Their parallel negotiations with Tehran over the nuclear programme are currently stalled.
At their last meeting, held last June in Moscow, Tehran rejected P5+1 calls for it to scale back its nuclear enrichment activities, while asking for substantial sanctions relief.
Iran's economy is struggling to cope with punitive measures adopted by the US and the EU targeting its vital oil income and access to global financial systems.
On Tuesday, top lawmaker Aladin Boroujerdi repeated Tehran's demands, the ISNA news agency reported.
"The main focus of the next talks with the P5+1 should be on lifting the sanctions imposed by the US and the European Union," said Boroujerdi, who heads parliament's influential foreign policy committee.
Mehmanparast added on Tuesday that a much-debated religious decree against nuclear weapons by Iran's supreme leader, Ayatollah Ali Khamenei, who has the final say on the nuclear programme, was binding.
"The fatwa is of utmost importance in clarifying Iran's nuclear activities," he said, dismissing Western doubts that the fatwa only served a religious purpose and was not necessarily an obligation for the government.