"The bus was riddled with bullet holes. There was broken glass everywhere," said an airport employee, who asked not to be named.
She said there were at least 25 people in the bus when it was fired on as it passed through a Baghdad suburb on Thursday.
The bus was carrying administrative employees, she said.
Fighters frequently attack buses transporting people to work at US-run bases or other installations seen to be under the control of US forces.
Meanwhile, a number of US soldiers were injured when explosive devices detonated on the main highway in al-Miqdadiya northeast Baquba, sources told Aljazeera.
The explosion severely damaged two US military vehicles, residents told Aljazeera.
In a separate development, two US military vehicles were destroyed when an explosive device detonated on the main highway in Karma area in northeastern Falluja, Aljazeera learnt.
Following the attack US forces sealed off the site and stormed a nearby house, arresting a man and two women.