“We must be able to verify the sites,” Fabius told BFM TV. “We don’t yet have certainty on this.”

“We want a deal with Iran but … the deal must be verifiable, solid, robust and today we don’t have guarantees on this,” he said. “A deal that cannot be verified cannot be implemented.”

Iran and six major powers reached a framework nuclear deal on April 2 in Lausanne, Switzerland, and are seeking to strike a final agreement by June 30 under which Iran would restrain its nuclear program in exchange for relief from economic sanctions.

“We are not yet at the end of the discussion,” Fabius said.

The United States also urged Iran to implement nuclear transparency measures outlined in a preliminary deal reached with world powers in April, in a statement issued in Vienna on Thursday.

Laura Kennedy, the U.S. envoy at the International Atomic Energy Agency (IAEA) in Vienna, told the IAEA board that it “remains critical for Iran to implement the provisions of Modified Code 3.1 … without delay.”

This provision requires from Iran early notification of the construction of new nuclear facilities.

Meanwhile, a U.S. official on Wednesday warned that the final stretch of international negotiations on curtailing Iran’s nuclear ambitions will be “pretty tough” as the weeks count down to a June 30 deadline for a historic agreement.

The long-sought deal — which is bitterly opposed by Israel — would row back Iran’s nuclear program in return for relief from punishing sanctions.

“As we expected after Lausanne (where a framework deal was struck on April 2), the next portion of this process will be pretty tough because we will be getting down to the details,” the U.S. official told reporters.

“You are all well familiar with the roller coaster ride of these negotiations and we all expected that this will get more difficult.”

Negotiators have until the end of the month to reach a comprehensive agreement limiting Iran’s nuclear program, which Tehran insists is for peaceful purposes but Western nations fear is aimed at acquiring if not a bomb, then the capability to build one.

Recent talks in Geneva failed to bridge differences between Washington and Tehran, especially over the crucial issue of inspections of military sites.

Other sticking points remain, including the possible military dimensions of the Iranian nuclear program and the demands by the world powers for U.N. inspections of Iranian military bases.