Iran's position in talks over its nuclear development programme is "inconsistent" with the terms of the deal to limit it, diplomats from the western European countries negotiating with Tehran said on Monday.
After a five-month pause, talks in Vienna on salvaging the 2015 nuclear deal between Iran and world powers resumed on November 29.
The 2015 agreement aimed to prevent Iran from developing a nuclear bomb, a goal Tehran has always denied.
The deal ensured sanctions relief in return for tight curbs on Tehran's nuclear programme, which was put under extensive UN monitoring.
Diplomats from Iran, Britain, China, France, Germany and Russia, 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. We are losing precious time dealing with new Iranian positions inconsistent with the [nuclear deal] or that go beyond it."
Former president 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 indirectly in the talks.
Iran wants Washington to lift sanctions and is asking for guarantees that they will not be reimposed.
On Sunday, Iran's chief negotiator at the talks, Ali Bagheri, reported progress on forming an agenda.
"The two parties are at the point of agreeing on the matters which should be on the agenda," Mr 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."
US and Israel open to 'other options' if diplomacy with Iran fails
Iranian officials maintain they are serious about committing to the talks.
But the western countries have accused Tehran of backtracking on the position it held earlier this year.
Last week, Mr Biden warned that the US was preparing "additional measures" against Iran as expectations grow that the talks are set to fail.
