The troops, part of an invasion force that stormed Iraq in March to topple President Saddam Hussein, fired back indiscriminately, residents said. "They were hitting houses and cars," said one man, Ahmed Amer.
One man said a woman was in hospital as a result of the shooting, which broke out at around 11 p.m. (3 p.m. EDT). Other residents said there had been no casualties.
U.S. military spokesmen said they no immediate information on any attack in Falluja, west of Baghdad, where several attacks have been mounted on U.S. troops since about 15 townsfolk were killed in clashes with the Americans in April.
Witnesses saw a building on fire and a resident showed reporters a car with its roof smashed and a hole in the plaster surface of a house wall, which he said was caused by U.S. fire.
Residents said the U.S. vehicles had come under grenade attack from assailants in a Volkswagen car that drove on. The soldiers opened fire on the surrounding area, they said.