The White House has described the letter as "reckless" and "irresponsible," saying it interfered with efforts by six major powers to negotiate with Iran and prevent it from building a bomb.

The negotiations, which resume in Lausanne, Switzerland, next week, are at a critical juncture as the sides try to meet an end of March target for an interim deal, with a final deal in June.

"We have been clear from the beginning, we are not negotiating, a quote, legally binding plan, we are negotiating a plan that will have in it capacity for enforcement," Kerry said. "The letter erroneously asserts this is a legally binding plan. It is not. We don't even have diplomatic relations with Iran."

"It is incorrect when it says that Congress can actually modify terms of an agreement at any time. That is flat wrong," Kerry said.

After the hearing Sen. Bob Corker, a Republican from Tennessee and chairman of the committee, said: "I want to follow up a bit on what he meant by that."

The letter was an unusual intervention by lawmakers into foreign policy, which is mainly the responsibility of the president's administration.