The Pentagon said that there are some Shi'ite fighters at an Iraqi military base where Washington just sent 450 reinforcements, but they were not working with US forces, underscoring concerns over links with the sectarian fighters.
Pentagon spokesman Colonel Steve Warren Tuesday said that while there is a "small group" of Shi'ite fighters at the Taqaddum base, there numbered only in the lower double digits, did not form full units and did not interact with US forces.
Taqaddum is a key area in the fight to retake the city of Ramadi and the Sunni province of Al-Anbar from the Islamic State group.
Shi'ite and Kurdish fighters have been critical in bolstering a weakened Iraqi military in efforts to fight IS. But the United States is hesitant to see Shi'ite militias involved in its efforts to battle IS, despite their battle readiness.
Many of the forces are backed by US-rival Iran and the military has not forgotten intense fighting with Shi'ite groups after the 2003 American invasion of Iraq.
The US and international observers also fret that such militias could spark sectarian fighting during campaigns.
In its annual report on terrorism released Friday, the US said Tehran continues to "sponsor terrorist groups around the world."
Warren said the Shi'ite fighters are coordinating with Iraq government forces at the strategic base, and that the US made it a condition of troop deployment to the airbase that Shi'ite militia units be withdrawn.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".
IS 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 IS hands.