Iran’s foreign minister said Tuesday a letter from U.S. Republican lawmakers warning that any nuclear deal could be scrapped once President Barack Obama leaves office suggests the United States is “not trustworthy.”

“This kind of communication is unprecedented and undiplomatic,” Mohammed Javad Zarif was quoted as saying by a state-run TV website, dismissing the letter as a “propaganda ploy” that “showed how worried one group is.”

“In fact, it implies that the United States is not trustworthy,” he added.

Senate Democratic Minority Leader Harry Reid called the letter a “hard slap in the face” designed to undermine the nation’s commander in chief “out of spite”.

The letter was an unusual intervention into U.S. foreign policy-making. It was signed by all but seven of the Republicans in the Senate and none of Obama’s fellow Democrats, who called it a stunt.

“It has come to our attention while observing your nuclear negotiations with our government that you may not fully understand our constitutional system,” the letter, which was signed by 47 Republican senators, begins.

The full letter can be read here.

Zarif linked the letter to Israeli Prime Minister Benjamin Netanyahu’s speech to Congress last week, in which the Israeli 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.

The U.S., along with five world powers, hopes to negotiate a framework agreement this month and a final accord later this year which would curb Iran’s nuclear program in exchange for sanctions relief.

Western nations have long suspected that Iran is covertly pursuing a nuclear weapons capability. Iran denies seeking nuclear weapons, and insists its program is entirely devoted to civilian applications like power generation and cancer treatments.

Republicans want any U.S. nuclear agreement with Iran to be approved by Congress. But Senate Majority Leader Mitch McConnell, who signed the letter released on Monday, agreed to postpone a vote on a bill requiring Obama to submit any deal for congressional approval amid outcry from Democrats.

Along with McConnell, Republican signers include Tom Cotton, Orrin Hatch, John Cornyn, John McCain, Lindsey Graham, John Thune and Mark Kirk. Ted Cruz, Marco Rubio and Rand Paul, two possible 2016 presidential contenders, also signed.

Reporting by Reuters and The Associated Press