Iran is ready to return to full compliance with a 2015 nuclear deal with major powers as soon as the other parties honour their commitments, President Hassan Rouhani said Wednesday.

The agreement between Iran and major powers has teetered on the brink of collapse since outgoing US President Donald Trump pulled out of it in 2018 and reimposed crippling unilateral sanctions.

US President-elect Joe Biden has expressed readiness to return to the agreement but over the past 18 months Iran has suspended the implementation of some of its own obligations, including key limits to its uranium enrichment programme.

"Just as soon as the 5+1 or 4+1 resume all of their commitments, we will resume all of ours," Rouhani said.

He was referring to the five veto-wielding permanent members of the UN Security Council plus Germany with whom Iran reached the nuclear deal.

"I've said it before -- it doesn't take time, it's just a question of willing," he said in comments to his cabinet aired by state television

Defying criticism from Iran's ultra-conservatives, Rouhani reiterated his determination to seize the "opportunity" presented by the change of US president in January.

Parliament, which has been controlled by conservatives since a February election marred by record low turnout, passed a bill last week that threatens the prospects for a thaw in relations with Washington.

The bill, which still has to be signed into law by Rouhani, would relaunch Iran's enrichment of uranium to 20 percent purity and threaten other future measures that would likely sound the death knell of the nuclear deal.

In a blow to the president, the Guardian Council, which arbitrates disputes between parliament and the government, approved the bill last week.

But in his comments on Wednesday, Rouhani appeared to suggest that he would withhold his signature from the bill.

"It is vital that we speak with a single voice," the president told ministers.

"People voted for a platform... and they want four years of action," said Rouhani, who won re-election in 2017 with more than 57 percent of the vote.

Rouhani said Iran remained unable to purchase Covid-19 vaccines because banks were unwilling to process the transaction for fear of falling foul of US sanctions.

Iran is the Middle Eastern country hardest-hit by the coronavirus pandemic with more than 51,000 deaths from nearly 1.1 million cases, according to official figures.

Vaccines and other humanitarian goods are supposed to be exempt from the US sanctions but in practice few if any banks are willing to take the chance. 

"We want to buy the vaccine... the money is... ready but no bank will handle the transaction," Rouhani told ministers.