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."

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.