US Secretary of State John Kerry (R) speaks during a Senate Foreign Relations Committee hearing on Capitol Hill in Washington March 11, 2015. [Photo/Agencies]

WASHINGTON - US Secretary of State John Kerry told Republicans who control Congress on Wednesday they would not be able to modify any nuclear agreement struck between the United States and Iran.

Kerry said he responded with "utter disbelief" to an open letter to Iran on Monday signed only by Republican senators that said any deal would only last as long as US President Barack Obama, a Democrat, remains in office.

"When it says that Congress could actually modify the terms of an agreement at any time is flat wrong," Kerry, who has been negotiating a deal to rein in Iran's nuclear program in exchange for easing sanctions, told the Senate Foreign Relations Committee. "You don't have the right to modify an agreement reached executive to executive between leaders of a country."

But Sen. Rand Paul, a possible Republican presidential candidate in 2016, told Kerry that any deal would need approval by Congress if it affected US sanctions against Iran. Paul accused the Obama administration of trying to bypass Congress.