WASHINGTON (AP) — Secretary of State John Kerry says U.S. lawmakers won’t be able to change the terms of any nuclear agreement with Iran because it won’t be legally binding.

His characterization could increase congressional opposition to a deal.

Kerry, America’s top negotiator with Iran, says he reacted in “utter disbelief” to a letter this week from 47 Republican senators warning Tehran that an accord with President Barack Obama’s team could expire the day he leaves office.

Testifying before the Senate Foreign Relations Committee, Kerry said Wednesday the letter undermined U.S. foreign policy and was legally incorrect.

Kerry said any pact would be among the executive branches of the seven governments negotiating right now — and the U.S. Congress can’t change it. Negotiators hope to reach a framework agreement by month’s end.

(TM and © Copyright 2015 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2015 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)