The British foreign secretary said the Republican US senators' letter to Iranian officials on a prospective nuclear deal could scuttle the negotiations between Iran and the major powers on a final settlement of the long-running dispute over Tehran's nuclear work.
The letter, released on Monday, said Iranian officials should be aware that any nuclear deal with US President Barack Obama could last only as long as he remains in office.
Philip Hammond warned that the letter "could become a spanner in the works" of the negotiations, due to resume on Monday, with an "unpredictable effect on opinion" inside Iran, the Telegraph reported on Thursday.
Former US secretary of state Hillary Clinton also on Tuesday excoriated the Republican senators for their letter, saying they might be trying to undermine the US commander-in-chief.
Monday's open letter, signed by 47 Republican senators, sparked a political firestorm. US Vice President Joe Biden also sharply criticized the lawmakers.
Clinton, a likely Democratic presidential candidate, said the Obama administration is in the midst of intense negotiations for a diplomatic solution to Iran's nuclear issue which would provide unprecedented access to its nuclear program.
"And one has to ask, what was the purpose of this letter?" Clinton said in an appearance at the United Nations, Reuters reported.
"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," Clinton said.
Biden said in a statement on Monday 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.
Many Republicans contend Obama is so eager for a nuclear deal that he would sign off on an agreement offering significant concessions to Iran.
Iran and the six major powers (the United States, Britain, France, Russia, China and Germany) are trying to reach a framework agreement this month and a final deal by June to place temporary constraints on Tehran's nuclear program in exchange for lifting sanctions. Iran says the program is for peaceful purposes only.
