Tehran (AFP) - Iranian Foreign Minister Mohammad Javad Zarif told top clerics Tuesday a letter from Republican senators undermining a possible nuclear deal had sapped Tehran's confidence in dealings with the United States.
Extending his criticism of the open letter, whose 47 signatories included several potential 2016 presidential candidates, Zarif said: "This kind of letter is unprecedented and undiplomatic. In truth, it told us that we cannot trust the United States."
Zarif's remarks, reported by the Isna news agency, came in Tehran at a meeting of the Assembly of Experts, Iran's top clerical body, where he gave an update on the negotiations with world powers for a comprehensive agreement on Iran's nuclear programme.
He said on Monday that the letter had "no legal value".
Effectively undercutting the White House, the senators wrote that President Barack Obama is in office only until January 2017, and a successor could scrap the agreement if Congress has not approved it.
While Iran and the United States are longtime foes, Zarif and his negotiating team have consistently said that the nuclear talks have been conducted in a good and serious atmosphere.
However he added Tuesday: "Negotiations with the United States are facing problems due to the presence of extremists in Congress."
The Republicans' letter appeared to be another bid to influence or even derail the talks between Iran and the P5+1 powers -- Britain, China, France, Germany and Russia as well as the United States.
It also marked a rare foray by Congress into foreign policymaking, as US negotiations with other governments are a responsibility typically handled by the executive branch, not lawmakers.
Obama pilloried the letter, comparing the senators to Iranian MPs who seem opposed to detente, saying he would make his case for any possible nuclear deal to voters.
"It is somewhat ironic to see some members of Congress wanting to make common cause with hardliners in Iran," he said.
With a March deadline looming, negotiators are furiously working to agree the political outlines of a deal that would curb Iran's nuclear programme in return for the lifting of Western sanctions.
The fine details of the accord are meant to be settled by the end of June but Iran's supreme leader, Ayatollah Ali Khamenei, has criticised the two-step process, saying matters should be handled in one sweep.
A new round of talks between Zarif and US Secretary of State John Kerry is due to take place in Lausanne, Switzerland on Sunday.