The trip is Rafael Mariano Grossi's first to the Islamic republic since the Argentine took the helm of the Vienna-based UN agency last year.

It comes more than two years after US President Donald Trump pulled out of a landmark international agreement that put curbs on Iran's nuclear programme in exchange for sanctions relief.

Grossi first met with the head of the country's atomic agency, Ali Akbar Salehi. Later he is due to meet Foreign Minister Mohammad Javad Zarif.

"A new chapter has started with this visit," Salehi said after the meeting, quoted by state news agency IRNA.

"Today's talks were constructive," he added.

"It was decided that the agency continue its work professionally and independently and Iran, too, act in the framework of its commitments."

In a statement before the meeting, the nuclear body had said Iran expects the IAEA to "maintain neutrality in any situation and refrain from entering international political games."

The UN agency's board of governors had passed a resolution in late June put forward by European states, urging Tehran to provide inspectors access to two sites to help clarify whether undeclared nuclear activity took place there in the early 2000s.

According to the spokesman for Iran's nuclear body, one of the two is located in central Iran between Isfahan and Yazd provinces, and the other is close to Tehran.

He hinted in a Monday interview with Iran's Al-Alam television network that access may be granted if the agency does not demand more.

"To prevent enemies from exploiting the situation... we are seeking ways to alleviate our concerns and say there is access, see there's nothing," Behrouz Kamalvandi was quoted as saying.

"But this issue must be resolved once and for all... meaning that they would not demand afterwards to inspect somewhere else in the same way," he added.

Access to the sites has been blocked for months, prompting a diplomatic row.

Iran has argued that the IAEA's access requests are based on allegations from Iran's arch-enemy Israel and have no legal basis.

- Rising tensions -

Grossi's visit comes amid tensions between the US and its European allies over Washington's bid to maintain an arms embargo on Iran and reimpose UN sanctions.

It also takes place shortly before a September 1 meeting of the joint commission on the 2015 deal between Iran and global powers.

The deal has been on life-support since Trump's decision to pull out of it in 2018.

The US and European nations are at loggerheads after Washington began the process Thursday of activating a controversial mechanism aimed at restoring UN sanctions against Iran.

Britain, France and Germany rejected the move, saying it frustrated their efforts to salvage the 2015 nuclear accord.

Washington controversially maintains it has the right to force the reimposition of sanctions through the agreement's "snapback" mechanism, despite its withdrawal.

Iran-US tensions have escalated since the US in May 2018 withdrew from the deal formally known as the Joint Comprehensive Plan of Action, or JCPOA.

In retaliation, Iran has progressively abandoned commitments under the accord that was meant to give it relief from sanctions in exchange for curbs on its nuclear programme.

Grossi is also expected to meet Iran's President Hassan Rouhani on Wednesday before flying back to Vienna.