They said the US soldiers opened fire on cars at the entrance to the town that has been the hotbed of Iraqi resistance against the occupation.
The US military has admitted shooting dead two Iraqis and wounding four others.
But Rafeh Issawi, a doctor at the hospital where the wounded were taken, said four people were pronounced dead from gunshot wounds.
A correspondent of the French news agency AFP reported seeing eight wounded Iraqis being taken to hospital. Locals said at least 11 civilians were wounded, including four seriously.
A doctor at the hospital where the wounded were taken said four people were pronounced dead from gunshot wounds.
Witnesses said that US soldiers opened fired about 10.45 pm (18:45 GMT) on cars at the eastern entrance to Falluja, some 50km west of capital Baghdad.
US forces barred the route and allowed only ambulances onto the site.
Witnesses earlier reported a man, woman and a child in extremely serious condition. It was not known if they were among the victims who died.
The incident fuelled anger among the local population, already hostile to the occupying forces.
Local mosques broadcast calls for blood donations, drawing some 2000 people to Falluja Hospital, where they chanted anti-US slogans such as "America is the enemy of God".
Under intermittent attacks in and around the town, edgy US soldiers have opened fired in Falluja at the slightest of provocations in previous weeks.
The worst incident to date was when US soldiers gunned down nearly a dozen Iraqi security men on Friday 12 September, apparently mistaking them for a hostile force.