A truce has brought a reduction in violence in the Iraqi city of Falluja after US troops fought insurgents there during a two-week siege.
The US says it has helped provide access to humanitarian aid
There was little media access during the fighting, but eyewitness reports are now emerging.
Humanitarian workers speak of US soldiers firing at ambulances and civilians.
They say makeshift clinics were overwhelmed because of a bridge closure which cut off access to the main hospital.
US military officials have described the US operation as "humane" and say they "do everything possible to protect non-combatants". But they say insurgents' tactics are increasing the risks for civilians.
Coalition forces began the operation to "pacify" insurgent fighters in the restive, mainly-Sunni city on 5 April. It followed the gruesome murder and mutilation in late March of four security contractors working for the coalition in the city.
The head of mission of a European humanitarian agency with staff in Falluja told BBC News Online that, according to his staff, two of their ambulances had been shot at.
"By who? The probability is by US snipers," he said.
SIEGE OF FALLUJA
Mon 5: 1,200 US troops seal off Falluja
Tues 6: Heavy fighting, US forces say they control industrial area on east of city
Wed 7: US forces bomb mosque compound, locals say up to 40 are killed, but US says no bodies found
Fri 9: Women, children and old men allowed to leave city. Relief workers take some aid in. Reports speak of bodies left in the streets. Fighting resumes in evening
Sat 10: Governing Council and US officials call for a truce
Sun 11: Tentative ceasefire begins at 0600. Thousands flee city
Mon 12: Negotiations continue through mediators for ceasefire to be extended
Fri 16: First direct negotiations between US officials and local leaders
Mon 19: Deal struck to reduce tensions, US announces it is 'halting operations', shaky ceasefire established
Asked whether these were warning or attacking shots, he said: "One was shot two or three times - a sniper does not shoot an ambulance three times by mistake."
British aid worker Jo Wilding said an ambulance she was in, with flashing lights, siren blaring and "ambulance" written on it in English, was hit as it drove to collect a woman in premature labour.
Ms Wilding is sure the shots came from American troops.
"You can tell the shape of US marine from a mujahideen - even if you can only see a silhouette, the helmet and flak jacket are quite distinctive. Also, we were in a US-controlled part of town," she told BBC News Online.
Iraqi doctor Salam al-Obaidi, a member of the Doctors for Iraq humanitarian society, worked in Falluja for six days during the fighting.
Speaking to BBC News Online, he described seeing colleagues blown up in an ambulance - also clearly marked - travelling in front of him as his team tried to enter a US-controlled area.
"I saw the ambulance disappear - not all of it, but the front of it, the side where the driver and paramedic were," he said.
He said he and two more colleagues were injured in a second explosion. He still does not know the fate of the two people in the first ambulance.
Some homes were destroyed in the fighting
In a separate incident, Dr Obaidi said, a driver and paramedic in an ambulance were shot in a US-controlled area - one in the chest, the other in the eyes.
The injured civilians inside the ambulance bled to death during the next two days as warning shots were fired when the team tried - four times - to return to collect the ambulance, he said.
Three days into the siege, Lt. Gen. Ricardo Sanchez, the top commander of U.S. forces in Iraq, denied that troops were firing on ambulances.
"If we're shooting vehicles, it's because those vehicles have shot at us," he said.
If this hospital was working it would have saved a lot of lives
Ibrahim Younis, Medecins Sans Frontieres
US officials have said that on one occasion, an insurgent gunman was seen fleeing in an ambulance, and that weapons have been found in an aid convoy west of the city.
Coalition military spokesman Brigadier General Mark Kimmit said that there have been "a lot of people running around the city with blankets on their vehicles asserting that they are ambulances".
There was concern that these could have been loaded with explosives, he said.
The Iraqi death toll from the siege has been strongly contested. Local doctors have been widely quoted as saying at least 600 people died.
Mr Obaidi believes the total to be at least 750, not including those buried in gardens or other unofficial grave sites.
The Iraqi Health Minister, Khodair Abbas, said on Thursday that 271 people died and local doctors had been pressured to give inflated figures.
Major roads into Falluja were blocked throughout the siege
The proportion of these who were civilians is impossible to verify.
Reports from the city have consistently said that many civilians in US-controlled parts of the city were too afraid of US snipers to leave their homes during the siege.
Dr Obaidi and Ms Wilding described cases of women, children and old men who appeared to have been shot by US soldiers.
Dr Obaidi said he had seen the bodies of two men, one aged about 70, the other about 50, both shot in the forehead, in an area controlled by the US.
They had been lying at the front gate of their home for two days, he said, because the family did not dare step outside to retrieve the bodies.
Is he sure they were shot by US troops?
"You are joking?" he said. "There are people dead in an area just controlled by America snipers. Nobody, either civilian or resistance, could enter the area. Who could kill them? We know American bullets. We are not a stupid people."
Ms Wilding said an injured mother and two children had told her they were hit by US gunmen as they tried to leave their house.
She also said she met an old woman, shot in the abdomen, who was still clutching a white flag.
"Her son said she had been shot by US soldiers," Ms Wilding said.
Tens of thousands fled the city during the fighting
Dr Obaidi also said he had seen the body parts of a family in a bombed-out house: "There were seven women and five children. I saw the head of a child away from the body. Only one girl, aged four, had survived," he said.
US officials say their operations have been "extraordinarily precise".
Gen. Sanchez said civilian casualties were "absolutely regrettable", but were a fact on a "battlefield of this nature in an urban environment".
Gen. Kimmit, also blamed militants who "hunker down inside mosques and hospitals and schools, and use the women and children as shields" for the civilian suffering.
The US has also faced criticism for blocking access to the city's main hospital by, according to most reports, occupying the river bridge which linked it to the rest of the city.
"If this hospital was working it would have saved a lot of lives," Medecins Sans Frontieres' Emergency Coordinator for Iraq Ibrahim Younis said.
Doctors set up makeshift clinics in the early days of the siege.
Ms Wilding said doctors were storing blood in a drinks fridge at a GP's surgery where they were treating the injured, and warming the bags under the tap in an unhygienic toilet.
Dr Obaidi said hundreds of patients were brought in, but his team had only 10 beds.
Part of the deal to end the fighting was a US commitment to allow "unfettered access" to the hospital and to "facilitate the passage of official ambulances" in the city.
The Coalition says troops "have consistently allowed food, medical and humanitarian supplies into the city" and have "assisted in the transportation and distribution of these supplies".
It also says marines have helped ambulances from Baghdad to get into Falluja, and that humanitarian convoys have been slowed by explosive devices found on the roads.