The Iraqi government has told the United Nations that militants have seized one of Saddam Hussein's former chemical weapons factories, confirming an earlier claim by Washington.

In a letter to UN Secretary General Ban Ki-moon dated July 1 and made public Tuesday, Baghdad's UN Ambassador Mohamed Ali Alhakim said "armed terrorist groups" entered the Muthanna project site on the night of June 11 after disarming the soldiers guarding it.

As a result, Baghdad was currently unable to "fulfill its obligations to destroy chemical weapons," Alhakim wrote, adding that "remnants of the (country's) former chemical weapons program" are kept at the site.

"The government will resume its efforts with regards to its obligations as soon as the security situation has improved and control of the facility has been regained," he added.

At dawn on June 12, the site's surveillance system, disabled by "the terrorists," showed there was "looting of some equipment and appliances," he wrote.

The letter confirms a June 19 claim by Washington that Sunni radicals had taken control of the facility.

State Department spokeswoman Jen Psaki said at the time that she didn't think the Islamic State of Iraq and the Levant (ISIL) militants would be able to produce usable chemical weapons there because any materials remaining there were old and unwieldy.

The militants have led a month-old crisis that has seen a jihadist-led alliance overrun large swaths of northern and north-central Iraq, displacing hundreds of thousands.

The complex, located just 45 miles (72 kilometers) northwest of the Iraqi capital, began producing mustard gas and other nerve agents, including sarin, in the early 1980s soon after Saddam took power, according to a CIA factsheet.

The program expanded to its height during the Iran-Iraq war later that decade, and produced 209 and 394 tons of sarin in 1987 and 1988 respectively.

But the CIA writes that the facility shut down after the first Gulf war, when UN resolutions "proscribed Iraq's ability to produce chemical weapons."

In the early 1990s, the site was used to oversee efforts to destroy Iraq's chemical weapons stockpile.