Shelling and fierce exchanges of gunfire broke out mid-afternoon on Wednesday in three districts of Falluja, which had already been pounded by airborne gunships overnight.
Local hospital staff said at least 10 residents had been wounded in the latest bout of fighting.
"Two US helicopters bombed the Golan and Askari districts in Falluja, just a short period after the Martyrs district was bombed by a jet fighter," Aljazeera's correspondent in Falluja said.
Aljazeera footage showed plumes of black smoke rising from several buildings that had been hit by US airborne gunships. The bombardment came hours after the US forces allowed some families to return to Falluja.
According to a US spokesman two resistance positions were targeted and a gun battle broke out near the railway station.
Amid warnings from the UN's envoy to Iraq of bloody consequences should talks fail in Falluja, an AC-130 gunship blasted Golan overnight as US forces attacked in what they said was a response to fire from Falluja's defenders.
From Washington, President George Bush warned his troops would take whatever action was necessary to secure the Iraqi city.
Bush said there were "pockets of resistance" in the Sunni bastion, some 50km west of Baghdad, but "most of Falluja is returning to normal".
Despite the clashes, the US military spokesman in Baghdad, Brigadier General Mark Kimmitt, told ABC television he still hoped for a deal.
"We are going to continue to push the political track as far as it is going to take us. If it does not take us far enough, we are prepared to use military means," he said.
Falluja has seen the fiercest fighting by the US-led occupation forces, with scores of US soldiers and hundreds of Iraqis killed there this month despite a fragile ceasefire.
Joint security patrols by US forces and Iraqi police in Falluja, already delayed, are scheduled to begin on Thursday.