Diplomatic sources close to the Iranian nuclear negotiations say progress has slowed considerably just weeks before a deadline on a historic agreement between Iran and six world powers.
The sources told Russian news agencies the slowdown is worrying because the final stage of negotiations is approaching with the June 30 deadline.
The latest round of the negotiations took place Friday in Vienna.
Iran has insisted that international inspectors will not be given access to the country's military sites as part of a new deal. Inspection of those sites is a key Western demand.
Darryl Kimball, director of the Arms Control Association told VOA that negotiators are trying to compromise on giving the International Atomic Energy Agency access to the sites.
"Iran is concerned that that access is not without some management, and the P5 plus one countries want to make certain that Iran can't keep sites off limits," Kimball said. "So they are discussing what is called `managed access' to sites of concern, including military sites."
Kimball said both sides are maneuvering to reach a viable agreement by the June 30 deadline.
"Both sides are going to be trying to find as much leverage as they can to get the best deal possible," said Kimball. "But fundamentally the agreement that they are working toward would block Iran's pathways to nuclear weapons: the uranium route, the plutonium route, the clandestine route. And put in place an unprecedented, layered monitoring program that would detect and deter a possible Iranian violation of the agreement for well over a decade."
In Washington, the State Department said Iran has to provide necessary access. "Iran has to reach agreement about the necessary access, and without that we will not be able to move forward with sanctions relief," said spokesman Jeff Rathke.
Negotiators from Iran and diplomats from the U.S., Britain, China, France, Russia and Germany have been working on details of a final agreement to scale back Iran's nuclear program in exchange for sanctions relief. They say the inspectors must have access to Iran's sites and scientists to determine if Iran is attempting to build an atomic bomb.
Iran insists its nuclear program is peaceful, with applications for medical research and power generation, but not for developing nuclear weapons.