Turkish Foreign Minister Hakan Fidan and his Iranian counterpart Abbas Araghchi have discussed the latest situation in the negotiation process with the US.
The two ministers held a phone call as Iranian media reported that the United States has failed to make any concrete concessions in its latest response to Tehran's proposed agenda.
The Fars news agency said Washington has presented a five-point list, which included a demand for Iran to keep only one nuclear site in operation and transfer its stockpile of highly enriched uranium to the US.
Washington also reportedly refused to release "even 25 per cent" of Iran's frozen assets or pay reparations for war damage.
The US has conditioned the cessation of hostilities on all fronts on the start of negotiations.
Iranian media have argued that the US is seeking concessions it failed to obtain during active combat, leading to an impasse.
In its own proposal, Iran has called for an end to the war on all fronts—including Lebanon—and a halt to the US naval blockade.
Tehran has also emphasised its continued management of the strategic Strait of Hormuz.
Regional tensions have escalated since the US and Israel launched strikes against Iran in February.
While a ceasefire has been in effect since 8 April, Iranian armed forces spokesperson Abolfazl Shekarchi warned Donald Trump on Sunday against restarting attacks.
"The desperate American president should know that if his threats are carried out... his country's resources and military will be confronted with unprecedented, offensive, surprising and tumultuous scenarios," he said.
