Iraqi officials said US forces killed up to six people in a raid on neighbourhood in west Baghdad late on Monday, but the US military declined to confirm any operation in the city.
An interior ministry source said six people were killed and three wounded in the raid, which the source described as an airstrike. Sources at Baghdad police headquarters said five were killed and 15 wounded in the attack on the Shula district, a rare Shia enclave in the mainly Sunni west of the capital.
State television, said nine people had been killed in a US airstrike in Shula.
US Lieutenant Colonel Christopher Garver said: "We can't confirm reports of ongoing operations at this moment."
Local residents in Shula said they heard loud explosions and gunfire throughout much of on Monday evening.
Washington has been pressing Prime Minister Nuri al-Maliki to crack down on militias linked to his political allies and blamed by Arabs for operating death squads.
But Maliki has insisted he needs time to forge a political consensus to reduce violence.
He criticised a US raid last month on the militia stronghold of Sadr City in east Baghdad where US troops were seeking a renowned warlord known as Abu Deraa.