A handout picture released by the office of Iran's Supreme Leader Ayatollah Ali Khamenei. Iran’s supreme leader expressed concern about U.S. internal rifts complicating the nuclear talks. (Ho/AFP/Getty Images)

Iran’s supreme leader expressed concern Thursday that nuclear talks with the United States could face new hurdles, saying a letter of warning from Republican senators signaled a “collapse of political ethics” in Washington.

Ayatollah Ali Khamenei suggested that internal divisions in Washington make Iranian negotiators question the U.S. ability to follow through with the current talks over ways to monitor and limit Tehran’s nuclear program.

“Isn’t this the ultimate degree of the collapse of political ethics and the U.S. system’s internal disintegration?” Khamenei was quoted by the official IRNA news agency in his first public statements since the GOP letter saying any possible accord could be undone unless it had congressional approval.

Khamenei, who has final say in all key Iranian decisions, also predicted additional complications ahead for the nuclear talks with the United States and other world powers.

“Of course I am worried,” Khamenei was quoted by Iran’s Mehr news agency, “because the other side is known for opacity, deceit and backstabbing.”

Iranian Foreign Minister Mohammad Javad Zarif described an open letter on a nuclear deal with Iran signed by U.S. Republican senators as a “propaganda ploy” from pressure groups afraid of diplomatic agreement. (Reuters)

[Read: GOP contenders for White House embrace hard line against Iran]

Talks between Iran and world powers are scheduled to resume Sunday in efforts to reach a general framework before the end of the month.

The central issue remains the extent of Iran’s uranium enrichment program. The West and its allies are concerned that Iran could eventually produce warhead-grade material. Iran insists it does not seek atomic arms, but does not want to rely on outside sources for nuclear fuel for peaceful reactors.

“Every time we reach a stage where the end of the negotiations is in sight, the tone of the other party, particularly the Americans, becomes harsher, harder and more aggressive,” IRNA quoted Khamenei as saying. “This is the nature of their tricks and deceptions.”

The Republican letter, signed by 47 senators, advised Iran’s leaders that any nuclear pact could be at risk of being rejected in the future unless lawmakers are allowed a voice on the deal.

The Obama administration has called the letter a provocative intrusion into foreign affairs and a blow to U.S. standing among the other negotiation partners, which include the permanent U.N. Security Council members plus Germany.

On a visit to Washington, Germany’s foreign minister, Frank-Walter Steinmeier, described the letter as “not very helpful” to the negotiations, which he said had entered a “delicate phase,” the Associated Press reported.

On Wednesday, Secretary of State John F. Kerry expressed “utter disbelief” at the GOP letter, but insisted that Congress has no ability to unravel a possible deal.

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