Washington—Sunni extremists in Iraq have occupied what was once Saddam Hussein's premier chemical-weapons production facility, a complex that still contains a stockpile of old weapons, State Department and other U.S. government officials said.

U.S. officials don't believe the Sunni militants will be able to create a functional chemical weapon from the material. The weapons stockpiled at the Al Muthanna complex are old, contaminated and hard to move, officials said.

Nonetheless, the capture of the chemical-weapon stockpile by the forces of the Islamic State of Iraq and al-Sham, known as ISIS or ISIL, the militant group that is seizing territory in the country, has grabbed the attention of the U.S.

"We remain concerned about the seizure of any military site by the ISIL," Jen Psaki, the State Department spokeswoman, said in a written statement. "We do not believe that the complex contains CW materials of military value and it would be very difficult, if not impossible, to safely move the materials."

The takeover underscores the chaos gripping Iraq and the possibility that the growing Sunni rebellion could further destabilize the Middle East. Not lost on U.S. government and military officials is the irony that the latest chapter in a war designed to strip Iraq of chemical weapons could see radical Sunni extremists take control of that same stockpile.