Islamic State fighters fired Grad rockets on American troops east of Tal Afar, a town still under control of the militants west of Mosul, according to an online statement from the group.
The American military had said earlier that two US service members had been killed and five injured during combat operations in northern Iraq on Sunday.
Islamic State's self-proclaimed "caliphate" effectively collapsed last month, when US-backed Iraqi forces completed the recapture of Mosul, the militants' capital in Iraq, after a nine-month campaign.