Japan's Prime Minister Shinzo Abe heads to Iran this week on a rare diplomatic mission, hoping to ease tensions between the Islamic Republic and ­Tokyo's key ally Washington.

Tehran is locked in a bitter standoff with the US after President Donald Trump withdrew from a landmark nuclear deal last year.

Washington has now reimposed sanctions and shifted troops to the region, putting military and economic pressure on Iran, including by forcing US allies like Japan to stop purchasing Iranian oil.

Japanese government officials say Abe will not present Tehran with a list of demands, or deliver a message from Washington, but instead will want to position Japan as a neutral intermediary.

That could prove useful, said Michael Bosack, special adviser for government relations at the Yokosuka Council on Asia-­Pacific Studies.

"Japan carries none of the historical or religious baggage of other potential mediators... [and] has demonstrated its willingness to go its own way on Middle East policy," he told AFP.

"These factors better position Abe for engagement with Ayatollah Khamenei and mean that Japanese-proposed options could allow hard-liners in Iran to entertain proposed offramps, without the potential fallout that could come from accepting 'Western' solutions."

Abe will meet President Hassan Rouhani and the country's Supreme Leader Ayatollah Ali Khamenei on the trip - the first time a Japanese prime minister has visited Iran since 1978, a year before the country's revolution.

Against that backdrop, ­Japan is hoping to lower the temperature, officials say, with Abe winning Trump's blessing for the mediation trip when the US leader visited Tokyo last month.

"We believe it is extremely important that, at the leadership level, we call on Iran as a major regional power to ease tension, to adhere to the nuclear agreement and to play a constructive role for the region's stability," Chief Cabinet Secretary Yoshihide Suga said ahead of the trip.

Iranian commentators said Abe could ferry messages between the two sides.

"Mr Abe's visit comes right after meeting Mr Trump in ­Japan, therefore the Americans are interested to use this channel," Ebrahim Rahimpour, a former deputy foreign minister, told Iran's Shargh daily ahead of the trip.