The landmark 2015 deal promised Iran sanctions relief in return for limits on its nuclear programme, but has been on life support since then US president Donald Trump pulled out from it three years later.

Iran's Foreign Minister Hossein Amir-Abdollahian pointed to tensions with four parties -- the United States, Britain, France and Germany -- to the accord also agreed with China and Russia.

He attacked recent "contradictory statements of American officials" which he said "are intended to aggravate the troubles in Iran and to gain points at the negotiating table".

"We have differences with America and three European countries that cooperate in this area with it," he told reporters.

London, Paris and Berlin criticised Iran in a resolution of the UN International Atomic Energy Agency last week.

But, Iran's top diplomat said, EU foreign policy chief Josep Borrell and the EU coordinator on the talks, Enrique Mora, "are trying to find a solution".

Amir-Abdollahian noted that two key problems need to be resolved -- questions posed by the IAEA, and economic safeguards that Iran demands.

The UN nuclear watchdog has demanded "technically credible" answers from Iran concerning traces of enriched uranium found on three Iranian sites.

On the economic front, Iran fears that a future US administration will call into question any agreement concluded with that of President Joe Biden.

"Americans say they don't know what will happen after the Biden administration," said the Iranian foreign minister, adding that a "change of government is not meant to undermine international agreements".

"The duration of foreign companies' contracts with Iran should be such that foreign investors can trust Iran to transfer their capital," he added.