Witnesses reported an intense tank and artillery barrage in the southern Shuhada district and industrial zone. US warplanes also bombarded the eastern corridor of the city late on Tuesday evening, residents said.
Aljazeera reported that heavy smoke was seen spewing over the industrial neighbourhood in the city.
The general hospital had received an undetermined number of casualties and panic swept the city.
The US military said the bombardment was in response to an earlier attack on US positions.
"Significant numbers of enemy fighters - up to 100 - are estimated to have been killed," the statement read.
"We are responding after being under fire. We are hitting enemy positions in the city since 1830 (1430 GMT). We are using aircraft and artillery fire," said US marine spokesman Lieutenant-Colonel TV Johnson.
Iraqis in Falluja protested US air
strikes on civilian areas of the city
However, Iraqi sources said it was civilians who were killed in the barrage on the city.
One ambulance driver said he had transported two dead and 15 wounded to hospital, while families fleeing the air raids said there were more corpses and wounded trapped in the zones under attack.
A central Falluja hospital official, Dr Muhammad Aboud, said the dead included an eight-year-old child and a 65-year-old man.
Aerial assaults condemned
A US air strike on Falluja on 1 September killed 20 people, prompting a mass demonstration by hundreds of protesters who condemned the killing of civilians by US warplanes and called on US soldiers to fight them on the battlefield.
The US military has said it is targeting "safe-houses" used by Jordanian fugitive Abu Musab al-Zarqawi and his supporters. However, Falluja residents point to the number of killed and injured women and children after every air strike.
In Baghdad, a US soldier was killed by small arms fire on Tuesday afternoon, the US military said. The latest US military death brings the military fatalities figure to 999, with over 6900 wounded, according to the Department of Defence.