WASHINGTON: The Pentagon said Tuesday that there are some Shiite fighters at an Iraqi military base where Washington just sent 450 reinforcements, but they were not working with U.S. forces, underscoring concerns over links with the sectarian fighters.

Pentagon spokesman Colonel Steve Warren said that while there is a "small group" of Shiite fighters at the Taqaddum base, there numbered only in the lower double digits, did not form full units and did not interact with U.S. forces.

Taqaddum is a key area in the fight to retake the city of Ramadi and the Sunni province of Al-Anbar from ISIS.

Shiite and Kurdish fighters have been critical in bolstering a weakened Iraqi military in efforts to fight ISIS. But the United States is hesitant to see Shiite militias involved in its efforts to battle ISIS, despite their battle readiness.

Many of the forces are backed by U.S.-rival Iran and the military has not forgotten intense fighting with Shiite groups after the 2003 American invasion of Iraq.

The U.S. and international observers also fret that such militias could spark sectarian fighting during campaigns.

In its annual report on terrorism released Friday, the U.S. said Tehran continues to "sponsor terrorist groups around the world."

Warren said the Shiite fighters are coordinating with Iraq government forces at the strategic base, and that the U.S. made it a condition of troop deployment to the airbase that Shiite militia units be withdrawn.

"There were Shia militias units on Taqaddum at one point," Warren added. But "one of the conditions for our arrival there was that these units move off Taqaddum airbase."

ISIS launched a brutal offensive last June that overran large areas north and west of Baghdad.

Iraqi forces have made gains in Diyala and Salaheddin provinces north of Baghdad, but much of the country's west is still in ISIS' hands.