John Kerry
John KerryReuters

Secretary of State John Kerry on Wednesday declared that U.S. lawmakers won't be able to change the terms of any nuclear agreement with Iran because it won't be legally binding, The Associated Press (AP) reported.

The comments came amid tensions between Democrats and Republicans over a letter to Iran’s leadership, written by 47 Republican lawmakers, warning that any nuclear deal signed with President Barack Obama will be void once he leaves office.

Kerry, Washington's senior representative in talks with Tehran, said he reacted with "utter disbelief" to the letter.

He told the Senate Foreign Relations Committee that the letter undermined U.S. foreign policy and was legally incorrect.

"We've been clear from the beginning: We're not negotiating a, quote, legally binding plan," Kerry told the panel, according to AP. "We're negotiating a plan that will have in it the capacity for enforcement. We don't even have diplomatic relations with Iran right now."

Kerry added that the letter, posted Monday by freshman Senator Tom Cotton of Arkansas, "ignores more than two centuries of precedent in the conduct of American foreign policy."

Whereas formal treaties require ratification by two-thirds of the Senate, "the vast majority of international arrangements and agreements do not," he said. "And around the world today we have all kinds of executive agreements that we deal with," he added.

Kerry, who will meet Iran's foreign minister, Mohammad Javad Zarif, next week in Switzerland for another round of talks, said the senators' letter "erroneously asserts that this is a legally binding plan. It's not, that's number one. Number two, it's incorrect when it says that Congress could actually modify the terms of an agreement at any time. That's flat wrong. They don't have the right to modify an agreement reached executive to executive between leaders."

Kerry also scoffed at the notion that Obama's successor would discard a deal reached between so many powerful governments and adhered to by Iran.

"I'd like to see the next president, if all of those countries have said this is good and it's working, turn around and just nullify it on behalf of the United States," he said, according to AP. "That's not going to happen."

The letter was fiercely criticized by Obama, who said on Monday, “I think it's somewhat ironic to see some members for Congress wanting to make common cause with the hard-liners in Iran. It's an unusual coalition.”

Vice President Joe Biden said in a statement on Monday night the letter was "expressly designed to undercut a sitting president in the midst of sensitive international negotiations" and was "beneath the dignity" of the Senate.

"This letter, in the guise of a constitutional lesson, ignores two centuries of precedent and threatens to undermine the ability of any future American president, whether Democrat or Republican, to negotiate with other nations on behalf of the United States," Biden wrote.

Former Secretary of State Hillary Clinton, a likely Democratic presidential candidate, lambasted the senators who wrote the letter and asked, "And one has to ask, what was the purpose of this letter?" "There appear to be two logical answers. Either these senators were trying to be helpful to the Iranians or harmful to the commander-in-chief in the midst of high-stakes international diplomacy. Either answer does discredit to the letters' signatories," she added.

Iran’s Foreign Minister, Mohammad Javad Zarif, claimed that the senators are not educated on international policy and even the United States constitution itself.