Ayatollah Ali Khamenei
Ayatollah Ali KhameneiAFP photo

Iran's supreme leader Ayatollah Ali Khamenei warned on Sunday that Washington was "not trustworthy", after former U.S. officials and lawmakers urged diplomacy with the Islamic republic's incoming president, Hassan Rouhani.

"I said at the beginning of the (Iranian) year that I am not optimistic about negotiations with the U.S., though in the past years I did not forbid negotiating (with them) about certain issues like Iraq," he told top officials at an iftar evening meal that breaks the daily fast during the Muslim holy month of Ramadan, according to the AFP news agency.

Khamenei said in March he was "not optimistic" over the prospects of direct talks with the U.S. on the sidelines of its nuclear negotiations with major powers.

"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 Rouhani 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 U.S. officials and dozens of American lawmakers called for President Barack Obama to pursue diplomacy with Rouhani.

In a letter to Obama, the ex-policymakers said the election of Rouhani, who takes office on August 3, "presents a major potential opportunity."

"We strongly encourage your administration to seize the moment to pursue new multilateral and bilateral negotiations with Iran once Rouhani takes office and to avoid any provocative action that could narrow the window of opportunity for a more moderate policy out of Tehran," they wrote, according to AFP.

"In interacting with the world it's a skill to continue your path without the other side being able to prevent you. If not you have lost," Khamenei added, alluding to Iran's future nuclear talks with world powers after Rouhani, who once served as his country's top nuclear negotiator, takes over from Ahmadinejad.

Rouhani, who has been described as a “moderate”, has indicated he would like a less confrontational approach to nuclear talks but at the same time he stressed election that Tehran would not consider halting the country’s uranium enrichment activities entirely.

Rouhani insisted that Iran’s nuclear activities are “within the framework of law,” and dubbed the international sanctions “baseless.”

He last week sent messages to Syria’s Bashar Al-Assad and Hizbullah reaffirming his support for the two allies, as well as his support of confrontation with Israel.

Obama at the start of his first term in office in 2009 offered talks with Iran, which has not had relations with the United States since its 1979 Islamic revolution which overthrew the pro-Western shah.

The United States has led a drive to cut off Iran's oil exports, its key source of revenues, as a way to pressure the regime over its controversial nuclear work.

Khamenei recently blamed the West for the standoff over the Islamic Republic’s nuclear program.

He said the standoff can be solved easily if the Western states stop their stubborn attitude.