The US has called on Iran's new president to return to talks on reviving a historic nuclear deal.

The window for diplomacy would not remain open forever, a spokesman for the US state department warned.

Tensions between the US and Iran have soared since 2018, when then-President Donald Trump pulled out of the nuclear deal and restored sanctions.

Ebrahim Raisi was sworn in on Thursday saying he would support "any diplomatic plans" to end sanctions on Iran.

"All illegal US sanctions against the Iranian nation must be lifted," he said.

Western countries accuse Iran of trying to build nuclear weapons - a charge Tehran denies.

The 2015 deal between Iran and six other countries - the US, UK, France, China, Russia and Germany - saw it stop some nuclear work in return for an end to sanctions hurting its economy. But Iran re-started banned nuclear work after Mr Trump pulled out of the deal.

Thorny negotiations have been taking place in the Austrian capital, Vienna, between Iran and other countries which are still part of the agreement to try to revive the frayed deal and lift sanctions. However the talks have been on hold for several weeks.

In comments following Mr Raisi's inauguration, US state department spokesman Ned Price told reporters: "We urge Iran to return to the negotiations soon so that we can seek to conclude our work."

"Our message to President Raisi is the same as our message to his predecessors... the US will defend and advance our national security interests and those of our partners. We hope that Iran seizes the opportunity now to advance diplomatic solutions," he said.

But he added: "This process cannot go on indefinitely."

Mr Raisi, a hardline cleric, has succeeded Hassan Rouhani, seen in the West as a relative moderate.

The 60-year-old is close to Iran's Supreme Leader, Ayatollah Ali Khamenei, and has been touted as a possible successor.

Among the major challenges he faces is a battered economy, which has led to growing discontent among ordinary Iranians, who have seen a sharp rise in the cost of living. Iran has also blamed US sanctions for an acute shortage of medicines.

Alongside the economic crisis, Iran is battling the coronavirus pandemic. It is the worst-hit country in the region and has recorded its highest number of cases for the past three days running.

Iran has also been rocked by a wave of street protests in the south-west, triggered in part by severe water shortages.

A former judiciary chief, Mr Raisi has been heavily criticised over his human rights record. Campaign groups have accused him of involvement in the executions of thousands of political prisoners in 1988.

Mr Raisi was one of four judges who sat on secret tribunals that came to be known as the "Death Committee" which condemned the prisoners to death. He has said the sentences were justified because then-Supreme Leader Ayatollah Ruhollah Khomeini had issued a fatwa, or religious ruling.

The US imposed sanctions on Mr Raisi in 2019 for alleged human rights abuses.