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 2015 landmark international agreement that put curbs on Iran's nuclear programme in exchange for sanctions relief.

Grossi first met with the head of Iran's atomic agency, Ali Akbar Salehi, and later held talks with Foreign Minister Mohammad Javad Zarif.

"A new chapter has started with this visit," Salehi said after their 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."

Salehi said Iran's "enemies will not rest" but noted that Tehran has "so far been able to manage the issue".

- 'Safeguards verification' -

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

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

The two sides were working on a statement which would be announced "in due time", Salehi added.

Grossi tweeted later that the two sides were working on "reaching an agreement on IAEA's safegaurds verification activities in Iran."

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

Aside from meeting Grossi, Zarif also spoke by phone on Tuesday to his French counterpart Jean-Yves Le Drian, according to the French foreign ministry.

Le Drian stressed the "unfailing determination of the E3 (Britain, France, and Germany) to preserve the Vienna agreement" reached in 2015 while calling on Iran to stick to its nuclear commitments, it said in a statement.

The UN agency's board of governors had passed a resolution in late June put forward by the 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.

- US 'snapback' bid -

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 nuclear agreement.

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

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

The deal has been on life-support and tensions escalating since Trump's decision to pull out of the deal formally known as the Joint Comprehensive Plan of Action, or JCPOA.

In retaliation, Iran has progressively abandoned nuclear commitments under the accord.

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