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

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 United States, 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.