TEHRAN, Iran -- Iran's supreme leader said Thursday that a letter from Republican lawmakers warning that any nuclear deal could be scrapped by the next U.S. president is a sign of "disintegration" in Washington.

Ayatollah Ali Khamenei called the letter a sign of "the collapse of political ethics and the U.S. system's internal disintegration," according to the official IRNA news agency. It was the first reaction to the letter by Khamenei, who has the final say over all major policies.

Khamenei said states typically remain loyal to their commitments even if governments change, "but American senators officially announced the commitment will be null and void after this government leaves office. Isn't this the ultimate degree of the collapse of political ethics and the U.S. system's internal disintegration?"

Khamenei said that whenever the talks approach a deadline, "the tone of the other party, particularly the Americans, becomes harsher, harder and more violent. This is part of their tricks and deceits."

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However, he said, "Iranian officials know what they are doing."

Khamenei has generally supported the talks, but he frequently expresses doubt over the true intentions of the U.S. and other Western nations.

The supreme leader said a "Zionist clown" had delivered a speech in Washington, an apparent reference to Israeli Prime Minister Benjamin Netanyahu's address to Congress earlier this month, in which he argued against the emerging agreement.

The U.S. and five world powers are negotiating with Iran to try and reach a framework agreement this month and a final accord in July which would curb Tehran's nuclear program in exchange for sanctions relief.

Western nations have long suspected that Iran is covertly pursuing a nuclear weapons capability. Iran insists its program is entirely devoted to civilian applications.

Secretary of State John Kerry himself blasted the Republican signatories to the letter during testimony Wednesday on Capitol Hill, expressing "utter disbelief" at the document, which he labelled factually "incorrect."

The 47 GOP senators told Iranian leaders in their letter that any agreement between the United States and Iran could be short-lived.

"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," the senators wrote.

"That is flat wrong," Kerry said Wednesday. "They do not have the right to modify an agreement reached, executive to executive, between leaders of the country."

He lambasted them for hypocrisy, saying, "To write to the leaders in the middle of a negotiation -- particularly the leaders that they have criticized other people for even engaging with or writing to -- to write them... is quite stunning."