Iran's supreme leader says a letter from U.S. Republican lawmakers warning that any nuclear deal could be scrapped by President Barack Obama's successor is a sign of "disintegration" in Washington.

The official IRNA news agency on Thursday quoted Ayatollah Ali Khamenei as calling the letter a sign of "the collapse of political ethics and the U.S. system's internal disintegration." 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.
He said  he was worried because the United States was known for "backstabbing", the Mehr news agency reported.

Khamenei added at a meeting with President Hassan Rouhani and senior clerics that whenever negotiators made progress, the Americans became "harsher, tougher and coarser", Mehr reported.

The letter signed by 47 Republican senators warned Iran that any nuclear deal made with U.S. President Barack Obama could last only as long as he remained in office, in an unusual intervention into U.S. foreign policy-making.

Mehr quoted Khamenei as saying: "Of course I am worried, because the other side is known for opacity, deceit and backstabbing."

"Every time we reach a stage where the end of the negotiations is in sight, the tone of the other side, specifically the Americans, becomes harsher, coarser and tougher. This is the nature of their tricks and deceptions."

The negotiations, which resume in Lausanne, Switzerland, next week, are at a critical juncture as the sides try to meet an end of March target for an interim deal, with a final deal in June.

Khamenei added that U.S. accusations of Iranian involvement in terrorism were risible. Khamenei also criticzed a speech to Congress by Israeli Prime Minister Benjamin Netanyahu this month that warned the United States it was negotiating a bad deal with Iran that could spark a "nuclear nightmare."