By Paul Richter, Jun. 9, 2015, Stripes.com

WASHINGTON (Tribune News Service) — Over the past 20 months, world powers seeking a nuclear deal with Iran have worked out tentative agreements to monitor and secure the country's known nuclear facilities to prevent it from building a bomb.

But as negotiations head for a June 30 deadline, diplomats are still at odds over a tougher issue: how to keep tabs on military bases and other sites where the Iranians might secretly work on a weapon.

The six-nation diplomatic bloc negotiating with Iran wants United Nations nuclear inspectors to be able to poke around so-called undeclared facilities if they suspect nuclear work is going on. Such "challenge inspections" raise sensitive issues of national sovereignty and pride that in the past have set off years-long struggles with Saddam Hussein's Iraq, North Korea and Iran itself.

With Iran's known enrichment facilities at Natanz and Fordow, and as a heavy-water reactor at Arak, under international oversight, the country's leaders would almost certainly look elsewhere to conduct any secret nuclear work, said Gary Samore, former nonproliferation advisor to President Barack Obama.

"It's the undeclared sites that are the real threat," he said.