Iran is said to be working out the details of its final legal arrangement with the so-called P5+1 -- the United States, United Kingdom, France, Russia, China and Germany -- to reduce its ability to produce a weaponized nuclear device in exchange for reduced sanctions, and talks restarted this week in Vienna, Austria.
"I’m not expressing optimism on one side or the other," Kerry told the Senate Foreign Relations Committee on Tuesday. "I remain agnostic, and questioning, even as we remain just about halfway through."
Kerry revealed that the negotiations were coming in at a point where Iran is operating with a so-called "break-out" of two months, meaning it would take that long to create one bomb's worth of material, not to build a weapon or have the capacity to deliver it.
But Committee Chairman Robert Menendez, D-N.J., said he didn't think the negotiators were aiming high enough.
"I don't think that we did everything that we've done to only get a six or 12 months lead time, because a deal that would ultimately unravel the entire sanctions regime for a six-to-12-month lead time is not far from where we are today," Mendez said.
Menendez argued that such a short window would not provide enough time for a sanctions program to get up and running the event that Iran moved to build a weapon, especially because the deal would involve the international community drastically reducing existing sanctions.
"With no sanctions regime in place...the only option left to the U.S. would be to either accept a nuclear-armed Iran or to have a military option," he said.
But Kerry denied that the P5+1 nations were aiming low.
"Nothing’s agreed until everything’s agreed," he said. "This is a mosaic that's going to have to be put together and I can assure you that we’re going to strive to get the longest period we can get in terms of break-out."