After a five-month pause, talks resumed on salvaging the Joint Comprehensive Plan of Action (JCPOA) in Vienna on November 29.

The 2015 agreement aimed to prevent Iran from developing an atomic bomb, a goal Tehran has always denied. The deal ensured sanctions relief in return for tight curbs on Iran's nuclear programme, which was put under extensive UN monitoring.

Diplomats from Iran, Britain, China, France, Germany and Russia, the signatory countries to the 2015 agreement, are attending the current talks.

"We have had many hours of engagement, and all delegations have pressed Iran to be reasonable," said the diplomats, from Britain, France and Germany.

"As of this moment, we still have not been able to get down to real negotiations," they added.

"We are losing precious time dealing with new Iranian positions inconsistent with the JCPOA or that go beyond it."

Donald Trump pulled the US out of the accord in 2018 and US President Joe Biden wants to negotiate Washington's return, with US diplomats participating at one remove from the main talks.

Iran wants Washington to lift a raft of sanctions, and is asking as well for guarantees.

- Iran sees progress -

On Sunday, Iran's chief negotiator at the talks, Ali Bagheri, reported progress on drawing up an agenda.

"The two parties are at the point of agreeing on the matters which should be on the agenda," Tehran's chief negotiator Ali Bagheri told the official IRNA news agency.

"It's a positive and important evolution since, at the start, they weren't even in agreement on the issues to negotiate."

Iranian officials maintain they are serious about committing to the talks.

But the Western countries have accused Tehran of having backtracked on the position it held earlier this year.

Last week, Biden warned that the United States was preparing "additional measures" against Iran as expectations grow that the talks are set to fail.