TEHERAN (BLOOMBERG) - Ultraconservative cleric Ebrahim Raisi swept to a landslide win in Iran's presidential election, potentially setting Tehran on a more hostile course toward the West as world powers attempt to revive a nuclear deal that could see it return to global oil markets.

While the 60-year-old's ascension could complicate efforts to restart the 2015 accord to limit the country's atomic activity, it's not expected to derail them because that policy is decided by Supreme Leader Ayatollah Ali Khamenei.

Mr Raisi, who was elected after the lowest voter turnout in a presidential election in the Islamic Republic's history, is seen as a favorite to one day succeed him.

With what appeared to be a nod to continuity, judiciary chief Raisi said he would listen to and work with the administration of outgoing president Hassan Rouhani until he takes office in mid-August.

"We will certainly tap the experience of the current government," Mr Raisi said in a state television address standing next to Dr Rouhani as initial results came in.

"I will sit down with the ministers and use their experience and views."

While Mr Raisi has previously said he would preserve the nuclear deal that Dr Rouhani helped seal, he's also indicated he doesn't want to make it Iran's central foreign policy concern.

Mr Raisi secured 17.8 million votes, or 62 per cent of the ballots cast. The only moderate candidate in the race, Mr Abdolnaser Hemmati, came third with 2.4 million ballots, Mr Jamal Orf, head of Iran's presidential election headquarters said in a statement, adding that 90 per cent of ballots have been counted so far.

With support from the highest levels of Iran's religious and military establishment, Mr Raisi's election means that all of Iran's state institutions and levers of power will be controlled by hardliners.

This comes after eight years under moderate Dr Rouhani, who was central to the nuclear accord that former President Donald Trump withdrew the US from in 2018.

Foreign Minister Mohammad Javad Zarif said "the foreign policy of Iran, which is based on consensus, will continue".

Dr Rouhani remains in office until August, giving diplomats in Vienna a window to revive the deal that lifted penalties on Iran's economy.

Those deliberations are now likely to extend well into the summer, two senior Western officials familiar with the process told Bloomberg News on Thursday, while Mr Zarif said there was "a good possibility" of an agreement before the end of Dr Rouhani's term.

The US exit from the deal empowered Iran's hardliners and principlists, who were always critical of the agreement and won control of Parliament last year. Millions of voters stayed home from Friday's election after most moderate and reformist candidates were disqualified from running.

"Apathy seems to be the problem. What was the reason for apathy? You will see differences of views in Iran," Mr Zarif said.

Mr Raisi was sanctioned in 2019 by the Trump administration, which cited his role in a deadly crackdown a decade earlier on protesters alleging vote fraud.

Turnout was 48.8 per cent, the lowest ever in the 42 years that Iran has held presidential elections, according to figures announced by Interior Minister Abdolreza Rahmani Razli on state TV.

Officials with strong security ties from the presidency of Mahmoud Ahmadinejad, who served before Dr Rouhani, could be also asked to run Iran's oil ministry.

A hardline shift there may complicate potential future relationships with European oil majors such as France's Total, which abandoned a US$5 billion (S$6.7 billion) project under the threat of Mr Trump's sanctions, and pivot instead to Russian and Chinese developers and the Islamic Revolutionary Guard Corps' own engineering firms.

Washington's withdrawal from the nuclear deal sent tensions soaring in the Persian Gulf, fuelling regional conflicts and prompting Teheran to abandon constraints on its nuclear programme contained in the pact and to enrich uranium close to the level needed for a bomb.

It's less clear what Mr Raisi will mean for Iran's policies in the region, especially its troubled relationship with Sunni rival Saudi Arabia.

His proximity to the Revolutionary Guards means his presidency could strengthen their footprint on the economy, though, and their influence on foreign policy decisions.

Human-rights groups say he was a presiding judge in mass executions of political prisoners in 1988, something he has never publicly addressed.