By Laura Smith-Spark, CNN

(CNN) Iran's Supreme Leader Ayatollah Khamenei on Thursday accused the six world powers involved in negotiations over Iran's nuclear program of "deception, trickery and backstabbing" in their dealings with Tehran, according to Iranian state media.

He also criticized a letter sent by 47 U.S. Republican senators to Iran's leaders, which threatened to scupper any deal if a Republican President is elected next year, Iran's official Press TV reported.

"Of course, I'm concerned because the other side is into deception, trickery and backstabbing," Khamenei is quoted as saying in a speech in Tehran.

He suggested that the letter was part of a U.S. strategy of last-minute reversal aimed at undermining a comprehensive deal covering Iran's nuclear ambitions, Press TV said.

"This is part of their ploys and tricks," said Khamenei.

Iran faces a March 24 deadline to reach a deal over its nuclear program. Several interim agreements have been made in recent months, though a long-term pact so far has been elusive.

The six world powers -- the United States, Russia, China, the United Kingdom and France, plus Germany, a group known as the P5+1 -- are seeking a deal that will ensure that Iran doesn't develop nuclear arms.

Officials in Tehran have publicly insisted they want a nuclear program for energy purposes, not to create atomic weaponry.

The Republican senators' decision to write to Iran's leaders has stirred up a political firestorm in the United States.

U.S. Secretary of State John Kerry denounced the letter Wednesday at a Senate Foreign Relations Committee hearing, calling it a breach of "more than two centuries of precedent" and factually incorrect.

Did 47 Republican senators break the law in plain sight?

"It purports to tell the world that if you want to have any confidence in your dealings with America, they have to negotiate with 535 members of Congress," Kerry said. "That is both untrue and a profoundly bad suggestion to make."

Sen. Tom Cotton, a freshman Republican from Arkansas, penned the letter, which asserted that a lasting agreement would need congressional support.

He wasn't shy about his attempts to undermine the Obama administration's negotiations with Iran and said Tuesday that the letter is "about stopping Iran from getting a nuclear deal."