A senior U.S. official says Iranian negotiators confronted their American counterparts about a letter from Senate Republicans warning that any nuclear agreement could expire the day President Barack Obama leaves office.

The official says the letter came up in nuclear talks Sunday between senior U.S. and Iranian diplomats. It was raised again in discussions Monday led by Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif.

The official wouldn't characterize Iran's position. But both Iranian and U.S. officials have criticized the letter written by freshman Sen. Tom Cotton of Arkansas and signed by 46 fellow GOP senators.

Kerry has said several times that the letter undermines U.S. diplomatic efforts and falsely claims Congress can change an executive agreement.

The official wasn't authorized to speak publicly on the matter and briefed reporters only on condition of anonymity.