A leading Democratic critic of the Iran nuclear deal warned Wednesday that the Obama administration will never want to reimpose sanctions against Iran because Iran has said in a letter to the United Nations that any new sanctions would free it from following the terms of the agreement.

Sen. Bob Menendez, D-N.J., used Wednesday's Senate Foreign Relations Committee hearing to read from a July 20 letter from Iran, which is on the U.N.'s website. In the letter, the Iranians assert that any reintroduction of sanctions lifted under the agreement would amount to "significant nonperformance, which would relieve Iran from its commitments in part of in whole."

A clearly frustrated Menendez first said he was angry that Iran's letter seems to spell out an aspect of the deal that the Obama administration refused to explain to him when he first asked Treasury Secretary Jack Lew how the sanctions' snap-back provision would work.

"So basically, what I tried to get from Secretary Lew, and I can't get from my own government, I have to read it from the government of Iran to understand what the agreement … was about," he said.

But Menendez also said his reading of the letter means that all sanctions against Iran will fall away, and that if Congress tries to set up just the possibility of new sanctions against Iran for failing to comply, Iran would be free from its commitments under the agreement. In that way, Menendez warned that Iran is preventing the U.S. from having sanctions options ready to go if Iran fails to comply with the agreement.

"And the language is pretty clear — that the sanctions that expire next year, that the Congress passed 99 to zero, at least in the Senate and overwhelmingly in the House … you're not going to be snapping back to that," he said. "And No. 2, the Iranians are saying if you just simply reauthorize it — with all the waiver options that the president has — they will consider it a violation."

Nicholas Burns, a former undersecretary of state in the Bush administration who led early negotiations with Iran over its nuclear program, generally supports the Iran deal as long the Obama administration and future presidents toughly enforce it. But when it comes to the letter Menendez cited Wednesday, he said he believed the Obama administration should challenge the Iranians' interpretation.

"I would hope that the administration would challenge that letter, and I don't think we would have to abide by it," Burns told the Senate panel.

But Burns also testified that any re-imposition of sanctions unilaterally by the United States would not bring Iran back into compliance or back to the negotiating table because the Europeans would likely break with the U.S. and the international sanctions regime would crumble.

Mark Dubowitz, executive director of the Foundation for Defense of Democracies, disagreed, arguing that U.S. sanctions are still the most powerful restrictions on doing business with Iran because the limits we have imposed on doing business with Tehran's central bank.

Menendez, co-author of the original sanctions legislations, had pressed Treasury Secretary Jack Lew last Thursday during another hearing on whether the Obama administration would support the renewal of sanctions after they expire.

"We said we would not re-impose nuclear sanctions if they live with a nuclear agreement," Lew told Menendez at the time.

Menendez has introduced a bill to renew the sanctions before they expire in 2016, and wanted assurances from Lew that the administration would support his effort. As a vocal critic of the deal, Menendez has argued that the Obama administration's points that the deal includes effective "snap-back" sanctions provisions are meaningless unless there is something to snap back to.

Charles Hoskinson contributed to this report.