Iran's Islamic Revolutionary Guards threatened Saturday to target US sites in the Middle East if its tankers come under fire, Iranian media reported, as Washington was left waiting for Tehran's response to its latest negotiating position.

"Any attack on Iranian tankers and commercial vessels will result in a heavy attack on one of the American centres in the region and enemy ships," the Guards said, a day after US strikes on two Iranian tankers in the Gulf of Oman.

US President Donald Trump had said on Friday he was expecting Iran's answer to Washington's latest proposal for a peace deal "supposedly tonight".

But if Tehran sent Pakistani mediators a response, there was no public sign of it, and Iran's Foreign Minister Abbas Araghchi reportedly questioned the reliability of US leadership.

"The recent escalation of tensions by American forces in the Persian Gulf and their numerous actions in violating the ceasefire have added to suspicions about the motivation and seriousness of the American side in the path of diplomacy," he said in a call with his Turkish counterpart, according to Iran's ISNA news agency.