DUBAI, March 12 (Reuters) - Iran's Supreme Leader hit out on Thursday at a letter by U.S. Republican senators threatening to undo any nuclear deal between Washington and Tehran, saying he was worried because the United States was known for "backstabbing," Mehr news agency reported.
Ayatollah Ali 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 criticized 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."
(Reporting by Sam Wilkin, Editing William Maclean)