US President Donald Trump tempered expectations of a Middle East deal by saying on Sunday he had told his negotiators not to "rush", even after both Tehran and Washington signalled signs of progress towards an agreement to end the war.

The United States and Iran have observed a ceasefire since April 8 while mediators push for a negotiated settlement, although Iran has imposed controls on Gulf shipping and the US has blockaded Iran's ports.

"Both sides must take their time and get it right," Trump wrote in the same Truth Social post, while slamming the 2015 nuclear deal that former president Barack Obama agreed with Iran.

"The Blockade will remain in full force and effect until an agreement is reached, certified and signed," he added.

Iran's Tasnim news agency said Sunday its information was that key clauses of a possible agreement remained "unresolved at this time", including the issue of frozen Iranian assets.

In Washington, Republican lawmakers close to Trump were among those expressing fears of an agreement favourable to Iran. The top Republican senator overseeing defence policy, Roger Wicker, said that agreeing to a "rumored 60-day ceasefire" with Iran would mean "everything accomplished by Operation Epic Fury would be for naught!"

US Secretary of State Marco Rubio, meanwhile, told The New York Times that an agreement with Iran had garnered regional support but a nuclear deal couldn't be achieved "in 72 hours on the back of a napkin".

"Right now, we have seven or eight countries in the region that are endorsing this approach, and we're prepared to move forward on this approach," he said.

Earlier Rubio had said a bargain could be struck to end the regional war as early as Sunday. (AFP)


Edited by Robert Kemp