One U.S. official, speaking on condition of anonymity, described Ramadi as “a powder keg” and said any use of militia has “got to be dealt with very, very delicately.”

    “There’s the potential it can go very, very badly,” the official said, without predicting such an outcome.

U.S. officials said Washington was deeply divided about the involvement of Shi’ite militias with links to Iran, a U.S. rival that has been expanding its influence throughout the Middle East. After spearheading the recapture of Tikrit, some Shi’ite fighters last month went on a spree of burning, looting, and violence in the Sunni Iraqi city, according to local residents.

“There are people in our government who see any involvement of Iran as anathema. There are others who say the Shi’ite involvement will promote sectarian violence. There are others who say that’s not true,” a second U.S. official said on condition of anonymity.