"This kind of communication is unprecedented and undiplomatic," Zarif said. "In fact it implies that the United States is not trustworthy," he added.
Zarif linked the letter to Benjamin Netanyahu's speech to Congress last week, in which the occupying regime’s leader argued against the emerging agreement. "A propaganda campaign has begun with Netanyahu's speech before Congress and this is their second ploy," Zarif said. "While there is still no agreement, a group is commenting on its nature."
"It is unfortunate that a group is opposed to reaching an agreement. We insist that a possible deal should be one where our people's rights are observed and we are certain that there are measures to achieve such a deal," he said.
The White House and Democrats in Congress also denounced the letter, accusing the senators of trying to sabotage the negotiations.
President Obama called it an attempt to undermine the administration's foreign policy.
In the letter, 47 Senate Republicans said that any deal is only an "executive agreement" that may not last beyond Obama's presidency, which ends on Jan. 20, 2017.
"The next president could revoke such an executive agreement with the stroke of a pen and future Congresses could modify the terms of the agreement at any time," said the letter pushed primarily by Tom Cotton, a freshman senator from Arkansas.
As the nuclear talks continue, White House spokesman Josh Earnest described the Republican letter as "the continuation of a partisan strategy to undermine the president's ability to conduct foreign policy".
The letter is essentially designed to "throw sand in the gears" of ongoing talks, Earnest said, and is not a "role that our Founding Fathers envisioned for Congress to play when it comes to foreign policy".
Obama, speaking after a meeting with European Council President Donald Tusk, noted there is not yet an agreement with Iran. If there is, the president said, "then we'll be able to make the case to the American people, and I'm confident we'll be able to implement it".
In recent weeks, the Obama administration criticized the House Republican leadership for inviting Netanyahu to address a joint session of Congress about Iran.
Some members of Congress have objected to the prospect that Obama might not submit an Iran agreement for congressional approval. They have also discussed the possibility of increasing sanctions on Iran, though Obama administration officials have said that move would hurt negotiations.
Criticizing that letter, Earnest noted that many signers simply oppose any nuclear agreement with Iran.
U.S. Vice President Joseph Biden said the letter was "expressly designed to undercut a sitting president in the midst of sensitive international negotiations”.
It was "beneath the dignity of the institution I revere”, Biden said in a statement.
Zarif said he was astonished by the letter, saying it suggests the U.S. lawmakers "not only do not understand international law but are not fully cognizant of the nuances of their own Constitution when it comes to presidential powers in the conduct of foreign policy".
The Iranian minister said, "in our view, this letter has no legal value and is mostly a propaganda ploy".
Many U.S. commentators noted that the letter, like the Republican invitation to Netanyahu, marked a dramatic break from the tradition that partisan politics should "stop at the water’s edge” and not spread into critical U.S. defense and security policy abroad.
The New York Daily News published photographs of some of the signatories on its front page, over the headline: "Traitors”.