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."

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.

During his meeting with Rouhani, Abe said "Japan will patiently continue its diplomatic efforts to ease tensions in the Middle East," according to a Japanese government official.

"The two leaders agreed to maintain their close communications," the official said.

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 explained to Rouhani Tokyo's plans to send Japanese warships to the Gulf of Oman to protect shipping there, while calling on Tehran to take "responsibility" for securing sea lanes in the region.

"This kind of policy is aimed at securing Japanese vessels' safety," government spokesman Yoshihide Suga said earlier in the day, 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."