The Red Cross, the last international aid agency working inside Baghdad, suspended its operations after one of its workers was critically wounded in an attack on a convoy attempting to resupply the city's hospitals.
As American troops rolled into the Iraqi capital, the International Committee of the Red Cross said the "extremely dangerous" situation in the city meant it could no longer deliver supplies to medical facilities which have been without water or electricity for three days.
A spokesman for the organisation said: "Our team has not been able move about Baghdad for the last 12 hours. Given the chaotic and totally unpredictable situation in the city, getting from one place to another involves incalculable risks."
The decision was taken after two vehicles carrying large Red Cross flags, which were attempting to reach one of Baghdad's main hospitals, came under fire on Tuesday.
A member of staff, Vatche Arslanian, 48, a Canadian logistics expert, was seriously wounded and had to be abandoned after colleagues trying to rescue him were forced back by gunfire.
Mr Arslanian, who had been part of a team delivering emergency supplies to hospitals and water treatment works, was last night officially listed as missing feared dead. The ICRC said it was not known whether the convoy had been deliberately attacked or had been caught up in crossfire between American and Iraqi forces.
Aid workers in the city said the fighting meant that civilians had been left injured and dying in the open because rescuers were being fired upon as they tried to help any wounded.
Roland Huguenin-Benjamin, part of the Red Cross team in Baghdad, said: "Casualties have been seen on the roads, on some bridges and there was no immediate possibility of evacuating them, for the reason that there was immediate fire as soon as anybody was trying to approach. The problem is the lack of respect for ambulances and respect for casualties – to give allow a minimum of security for people to be evacuated."
The suspension of the ICRC deliveries will cut off the sole source of fresh supplies of medicines and equipment to Baghdad's four main emergency hospitals.