Rouhani's trip to Japan comes after deadly protests last month over petroleum price hikes in Iran, where the economy has been hit by Washington-imposed sanctions.

"I strongly condemn the US for unilaterally and irrationally withdrawing" from the deal, said Rouhani, who became the first Iranian head of state to visit Japan for two decades.

"I hope Japan and other countries will make efforts to maintain this deal."

Rouhani and Abe's summit talks and a dinner were scheduled to last into Friday evening. The two leaders are not scheduled to brief reporters after the talks.

The US re-imposed crippling sanctions on Iran in 2018 after withdrawing from the international deal aimed at tackling the Islamic Republic's nuclear programme.

Iranian government spokesman Ali Rabiei has played down the mediation aspect of the trip, saying the visit to Tokyo had "nothing to do with issues such as negotiations with America".

However, he acknowledged that "our Japanese friends usually convey messages or initiatives, which we welcome... and seriously examine".

As a key US ally that also maintains close diplomatic and economic ties with Iran, Abe has tried to build bridges between the two rival powers.

Last week, Abe said he would strive "as much as possible to ease tensions" in the Middle East, noting Japan's alliance with Washington and "favourable relations" with Tehran.

Abe travelled to Tehran in June to try to ease tensions between the United States and Iran in the Gulf.

Japan was formerly a major buyer of Iranian crude but stopped purchases to comply with the US sanctions.

Abe is expected to explain to Rouhani Tokyo's plans to send two Japanese warships to the Gulf of Oman to protect shipping there.

"This kind of policy is aimed at securing Japanese vessels' safety," said government spokesman Yoshihide Suga, adding that 90 percent of Japan's crude oil imports come from the region.

- 'Serious impact' -

Rouhani flew to Japan from Malaysia where he called on Muslim countries at a summit to band together to fight US "economic terrorism".

Osamu Miyata, head of Center for Contemporary Islamic Studies in Japan (CCISJ), told AFP that Abe would find it difficult to steer a path between US President Donald Trump and Rouhani.

The American sanctions "are having a serious impact on every aspect of Iran -- people's daily life, the country's finances, and inflation in imported goods", Hitoshi Suzuki, a Middle East scholar at the Institute of Developing Economies (IDE-JETRO), told AFP.

"It would be difficult to have tangible achievements from the Rouhani-Abe meeting this time, but in the long-term, Japan can warn the US that the current sanctions are having a serious negative impact," added Suzuki.

"This could prompt Iranian domestic politics to move in the opposite direction hoped for by the US -- for example, hawks leading Iran to resume nuclear development, or the emergence of an anti-democratic Iran."