"We are still not in a position to conclude that all nuclear material in Iran is in peaceful activities," Yukiya Amano, director general of the International Atomic Energy Agency said Monday during a visit to Washington.
Administration officials have insisted they will not sign a deal that does not prevent Iran from obtaining a nuclear weapon and does not contain mechanisms to verify that fact.
But as the talks reach a critical phase, with the U.S. negotiating team set to meet the Iranians next on Thursday, concerns have been raised about the reported outlines of the deal taking shape between Tehran and the "P5+1" countries — the United States, Britain, France, Germany, Russia and China.
International negotiators had already conceded that Iran would be allowed to enrich uranium to a limited extent, and reportedly also were looking at allowing any deal to expire in 10 years. There's also no sign that Tehran will be expected to give a full accounting of its past nuclear weapons research, which it has refused to do.
Under those circumstances, experts say, Iran cannot be prevented from cheating on any agreement.
"The difficulties are considerable. And they're made worse by what has been reported about the shape of the deal," William Tobey, a senior fellow at Harvard's Belfer Center for Science and International Affairs, told House lawmakers Tuesday.
David Albright, president of the Institute for Science and International Security, said Iran's history of evading constraints on its nuclear program that it committed to as a signatory of the Nuclear Non-Proliferation Treaty makes verification crucial to the success of any deal.
"A critical question will be whether the agreement establishes a verification regime adequate to promptly catch Iranian cheating," he said. "If no concrete progress on this issue is not forthcoming by July 1, a deal should not be signed."
Secretary of State John Kerry and other administration officials have insisted that any deal they sign will not be based on trust. But one of many problems they're having with selling a potential deal to a skeptical Congress is the fact that most lawmakers don't trust the administration's negotiating strategy, fearing it already has given too much to Iran for the sake of a deal.
The experts' concerns are bound to feed that perception, along with lawmakers' demands for a tougher stance that might force Iran to walk away from the talks.
"The only way we can ensure that Iran is in compliance with its [international] obligations and is not paving the way toward a nuclear weapon is to fully dismantle its infrastructure," said Rep. Ileana Ros-Lehtinen, R-Fla., chairwoman of a House Foreign Affairs subcommittee which examined the issue on Tuesday.
Experts agree that the key to adequate verification begins with Iran fully complying with International Atomic Energy Agency demands that it come clean about its past nuclear weapons research and allow intrusive inspections of military sites.
"I, for one, would not be comfortable with any agreement that does not force Iran to come clean about its past activities," said Rep. Ted Deutch, D-Fla., the subcommittee's ranking Democrat.
Other concerns include the length of the agreement. Albright, a former International Atomic Energy Agency inspector in Iraq, said 10 years would not be enough given Iran's past history of cheating, and suggested any deal last for 20 years or more.
"We need time in order to verify that they're reformed," he said, suggesting that keeping United Nations sanctions in place on dual-use technologies and equipment would help prevent covert nuclear activities.
Added Tobey: "I'm dumbfounded that we'd have an agreement that could be shorter than the time it's taken to negotiate it."
There's also the concern that the administration's goal of having an agreement, which would put Iran one year away from being capable of building a nuclear weapon if it does cheat, does not give the international community time to thwart the effort.
"Infractions of arms control agreements are difficult to prosecute," said Ray Takeyh, an Iran expert at the Council on Foreign Relations.
Takeyh co-authored a Washington Post op-ed published Monday with former CIA Director Michael Hayden and Olli Heinonen, a former deputy director general of the International Atomic Energy Agency, suggesting that Iran could cheat faster than the world could agree on how to react.
That's quite possible if the verification regime isn't set up to catch undercover nuclear activities, said Albright.
"You could easily have a situation with Iran where you do spend a year arguing," he said.