WASHINGTON — A plethora of new and old Shiite militias is dominating battlefields in Iraq and Syria, US experts say, raising concerns about growing Iranian influence that could exacerbate sectarian divisions and ultimately weaken the fight against the group that calls itself the Islamic State (IS).

The growth of these groups — known as popular mobilization units — poses a conundrum for the Obama administration and the coalition of about 60 countries it is leading against IS. For the time being, Iran-backed groups are providing most of the “boots on the ground” that are pushing back IS forces from key areas near Baghdad, including Diyala province. These militias are also preventing IS from overrunning even more territory in Syria. But there are disturbing reports of massacres of Sunni civilian populations by Shiite fighters, some of whom have killed Americans in the past.

At his confirmation hearing Feb. 4, Ashton Carter, the nominee to replace Chuck Hagel as defense secretary, went so far as to equate Iran and IS as the biggest threats to US national security, in part because of spreading Iranian influence in Iraq. "I have concerns about the sectarian nature of Iran’s activities in Iraq,” Carter explained to the Senate Armed Services Committee.

"The United States must continue to make clear to the Iraqi government that Iran’s approach in Iraq undermines the needed political inclusion for all Iraqi communities, which is required to ultimately defeat [IS]," he said.