The visit comes before the quarterly meeting of the International Atomic Energy Agency's board of governors next week, with efforts to revive a 2015 deal to halt Iran's nuclear development stalled.

Director-General Rafael Grossi will meet the Iranian vice president and the head of the country's nuclear agency, Mohammad Eslami, the UN body said in an emailed statement on Saturday.

He is expected to leave in the afternoon, Iranian ambassador to international organisations in Vienna Kazem Gharib Abadi tweeted.

Grossi -- on his second trip to Iran this year -- was expected to hold a press conference on his return to Vienna airport around 8:30 pm (18:30 GMT), the statement added.

The IAEA made a stark assessment on Iran in a report to member states this week.

Since February 23 "the Agency's verification and monitoring activities have been seriously undermined as a result of Iran's decision to stop the implementation of its nuclear-related commitments", the report said.

In these conditions, the IAEA's technical capacity to monitor Iran's nuclear programme has "significantly further declined", and its confidence "that it can maintain continuity of knowledge is declining over time", it said.

Iranian President Ebrahim Raissi insisted that his country was being "transparent".

"Naturally, in the event of a non-constructive approach by the IAEA, it is unreasonable to expect Iran to respond constructively," he said on Wednesday, according to a statement from the presidency.

In the meantime, talks begun in April in Vienna, under the aegis of the European Union, to try to resurrect the 2015 international agreement have been suspended since June.

No date for a resumption has yet been announced despite repeated calls from the West, which is alarmed by the recent revival of Iran's nuclear programme.

Faced with this impasse, the United States is "close" to abandoning its efforts, Secretary of State Antony Blinken said on Wednesday.

The US envoy for Iran Robert Malley was in Moscow and Paris from Tuesday to Friday for "consultations" with Russia and the Europeans.

He mentioned on Twitter "constructive meetings" with a view to a "rapid return" to the "Joint Comprehensive Plan of Action" (JCPOA).

The Islamic Republic has gradually freed itself from its nuclear obligations since 2019, in response to the reinstatement of US sanctions by former US President Donald Trump.