A spokesman for the US occupation military confirmed on Monday that two US civilians had been wounded and said three people had been killed in the attack, but declined to give details of the incident.
US military helicopters ferried the wounded to hospital after the attack.
Witnesses said the victims had been driving through Mosul in a civilian vehicle without a military escort. One US military source said the deceased "were possibly working for a non-governmental organisation."
"Two women and a man were killed, and a man and woman were wounded," said police officer Abd al-Amir Muhammad Jasim. Doctor Sultan Jargis at Salam hospital said the wounded American woman was hit by four bullets in the chin, leg and chest. Two bullets struck the wounded man in the stomach, he said.