Experts from the International Atomic Energy Agency (IAEA) were back in Tehran early Wednesday to try to 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.

It was not clear who would represent Iran in meetings on Wednesday, but the office of top nuclear negotiator Saeed Jalili is in charge of decisions regarding Tehran's atomic programme -- on behalf of supreme leader Ayatollah Ali Khamenei who has final say on all key state matters.

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 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.

Iran's foreign ministry spokesman said on Tuesday that the government hoped to conclude a comprehensive agreement with the IAEA on Wednesday.

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.

But the IAEA has pointed to new information uncovered since its last visits to the site in 2005.

They include satellite evidence that the earth has been scraped and removed over a 25-hectare (62-acre) area, leading to Western accusations that Iran is destroying evidence.

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 in June 2012 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, ISNA 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, 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.