Treasury Secretary Jack Lew on Thursday would not commit the Obama administration to supporting renewal of existing congressional sanctions on Iran after they expire in 2016.

When pressed by Sen. Bob Menendez of New Jersey, a leading Democratic critic of the nuclear deal under which those sanctions would be waived, Lew responded: "We said we would not reimpose nuclear sanctions if they live with a nuclear agreement."

Under the agreement reached July 14 in Vienna, Iran will receive relief from international sanctions that have crippled its economy in exchange for freezing its nuclear program for 10 years. If Iran violates the deal, the sanctions would "snap back" into place.

Menendez, co-author of the original sanctions legislation, has introduced a bill to renew them before they expire, and wanted assurances from Lew that the administration would support his effort.

"My point is this. If you're going to snap back, you've got to snap back to something," Menendez said, interrupting Lew to note that if the existing sanctions expire "there is nothing in that context to snap back to.

"So why won't you simply say that the administration supports under all the same provisions including the president's waivers reauthorization of those sanctions so that the Iranians know if they violate, that the snapback will also include snapback to what the Congress passed?" Menendez asked.

"Right now the sanctions remain in effect," Lew said, and continued: "It's premature to talk about extending a law…." But Menendez cut him off saying Iran's obligations go beyond 2016 under the agreement.

"I don't understand how we ultimately have a credible belief that snapback means something if in fact you're not going to be able to have those sanctions in place," he said.