Iran's supreme leader Ayatollah Ali Khamenei has warned that US is "not trustworthy", after former US officials and legislators urged diplomacy with Iran’s incoming president Hassan Rowhani.

“I said at the beginning of the [Iranian] year that I am not optimistic about negotiations with the US, though in the past years I did not forbid negotiating [with US] about certain issues like Iraq,” he said on Sunday during an Iftar meal that breaks the daily fast during the Muslim month of Ramadan.

“The Americans are not trustworthy and they are not honest in their encounters. The stance of American officials over past months once again confirms that one should not be optimistic," he said at the Iftar, attended by Rowhani and outgoing President Mahmoud Ahmadinejad.

The comments from Khamenei, who has the final say in the regime's macro policy issues, came less than a week after former US officials and dozens of American legislators called for President Barack Obama to pursue diplomacy with Rowhani.

In a letter to Obama, the ex-policymakers and some legislators had insisted that election of Rowhani, who takes office on August 3, “presents a major potential opportunity” and that “Rowhani should be tested”.

US legislators Republican Charles Dent and Democrat David Price have led a call for Obama to “utilise all diplomatic tools” with new Iranian president.

Ever since his election, Rowhani has vowed to engage constructively with the international community and to ease tensions over the country’s nuclear programme.

The US has not had relations with Iran since 1979 Islamic revolution which overthrew the pro-Western government even as US has led a drive to cut off Iran's oil exports, its key source of revenues, as a way to pressure Tehran to give up its nuclear programme.

US and its allies believe the programme is being used to develop an atomic bomb, a claim Iran denies.