The 450 U.S. troops who are settling into their new outpost at Iraq's Taqaddum Air Base are sharing the facility with some Shiite militiamen — but only a few.

Not long ago, Taqaddum was home to a large contingent of Shiite militia fighters, some of them likely linked to groups that were targeting U.S. troops in combat operations a decade ago.

But that changed after American military commanders recently identified Taqaddum, a sprawling base in the heart of Anbar province, as a good location for a new forward operating base for about 450 U.S. troops who will help support the Iraqis' fight against the Sunni extremist group known as the Islamic State group.

Top U.S. military officials told the Iraqis that the irregular Shiite militias, along with any Iranian military units, had to leave before American service members would arrive as part of the latest buildup of U.S. forces.

"One of the conditions for our arrival there was that these units move off of Taqaddum Air Base," Army Col. Steve Warren, a Pentagon spokesman, said Tuesday.

U.S. officials also specifically told the Iraqis to remove any units or leaders from the Iranian Revolutionary Guard Corps, or IRGC, Warren said.

The IRGC is a faction of Iran's military that is providing training and support to the Iraqi military in its fight against the Islamic State group.

For now, the Shiite militia presence at Taqaddum is limited to "some individuals who are working or serving in some sort of a liaison capacity" with the Iraqi army units at the base, Warren said.

"The [Shia militia] representatives who are on Taqaddum and the American forces who are on Taqaddum are separated by space. The government of Iraq is helping to coordinate the separation of these two groups," Warren said.

Just a couple of months ago, the top U.S. commander in the Middle East said Iraq's Shiite militias have American blood on their hands and he hoped today's mission to defeat the Islamic State extremists would not involve an alliance with those groups.

"Three tours in Iraq commanding troops who were brutalized by some of these Shia militias ... I will not — and I hope we never — coordinate or cooperate with the Shia militias," Army Gen. Lloyd Austin, chief of U.S. Central Command, told the Senate Armed Services Committee on March 26.

For instance, in March, the U.S. refused to provide airstrikes in support of an anti-ISIS operation in Tikrit that was led mainly by Shiite militias and some Iranian military officials.

American officials are wary of Shiite militia fighting in predominantly Sunni regions of Iraq for fear they will mistreat the local civilians and fuel sectarian tension in Iraq.

U.S. officials say sectarian tension is fueling support for the Islamic State group, which appeals to local Iraqis as a defender of Iraq's Sunni community against a repressive Shiite-led government in Baghdad and aggressive Shiite militias.

But after the demoralizing fall of the city of Ramadi to Islamic State forces in May, the U.S. military softened that stance and now agrees to support any military forces that are "under the command and control" of the Iraqi government.

These irregular forces that are not formally a part of the Iraqi army but operate under Iraqi commanders are known as "popular mobilization forces."

Today, U.S. aircraft can provide air support to ground-level Shiite militias fighting Islamic State forces. But apparently, sharing a base with those same units is a bridge too far.

"As we conducted our broader force protection analysis, we felt that it was in our interest, in the interest and safety of our personnel, to have these militia units move off of al Taqaddum Air Base," Warrren said.

The majority of the 450 U.S. troops who are setting up the American outpost at Taqaddum are dedicated to force protection. A small portion, potentially fewer than 100, will be mounting an advise-and-assist mission with the Iraq military leaders there.

The White House on June 10 announced plans to deploy an additional 450 troops to Iraq. That could bring the total number of U.S. forces to 3,500.

Those Americans who have begun arriving at Taqaddum recently welcomed about 500 Sunni fighters to a ceremony where the locals were officially inducted into a "popular mobilization" unit, and given pay and arms, said Marine Brig. Gen. Thomas Weidley, who is the chief of staff for the war effort in Iraq and Syria.