One of the grisliest tasks facing the authorities in New York will be the disposal of the remains of those who died.
With roads in lower Manhattan impassable yesterday, temporary morgues were set up on piers next to the Hudson river. Barges were used to carry bodies across the water.
Public health experts said there was no immediate threat of disease, given New York's secure water and sewage systems. However, the practical issue of how to handle the disposal sensitively and humanely would be immense.
At least 1,100 casualties had been taken to the 170 hospitals in New York by yesterday evening, but hospital authorities said they could have coped with many more. A spokeswoman for Beth Israel Hospital in Manhattan's East Village said: "There is a real feeling of pain here that we cannot do more. We want to do more."
More than 200 of the casualties were said to be in critical condition, with burns, smoke inhalation, cuts and broken bones. A further 2,000 "walking wounded" were shipped to Liberty State Park in New Jersey by ferry and tug boat.
In Washington, hospitals were reported to have treated more than 40 casualties from the attack on the Pentagon.
In New York, a firefighter, Rudy Weindler, spent nearly 12 hours looking for survivors and only found four – a pregnant woman sitting on a kerb and three others in the rubble of a building in the Trade Centre complex. "I lost count of all the dead people I saw," Mr Weindler said. "It is absolutely worse than you could ever imagine."
Angelo Otchy, a mortgage broker who arrived with a National Guard unit from Dover, New Jersey, to help dig through the debris, said: "I must have come across body parts by the thousands."
Offers of help came in from around the world. A Belgian team of 10 people that specialises in helping with severe burns as well as identifying bodies flew to Iceland to await further instructions from the United States authorities.
The European Union mobilised search and rescue teams. France had 250 people standing by, with a further 250 available, and Sweden, Ireland and Spain were also offering search and rescue assistance. The Dutch had a team of identification experts standing by.
In Manhattan, people started queuing to give blood from 6.30am. Hospitals reported that hundreds had turned up wanting to contribute to the rescue effort. One man outside Beth Israel Hospital said: "I saw it on television and I was just so shocked, I was crying. This is all I can do."
The National Association of Community Blood Centres, which supplies 70 per cent of the blood in the city, said it had sent 15,000 pints immediately after the attacks and was working with the military to provide more. A spokeswoman for the association said there were a large number of burns victims who would need platelets, plasma and red blood cells.
St Vincent's Hospital, the closest to the World Trade Centre, said it dealt with 360 patients immediately after the attack, including 54 from the emergency services. Fifty were seriously injured and five had died.
Professor Brian Duerden, the head of Britain's Public Health Laboratory Service, said: "There is some concern, but it is not an immediate concern. Dead human bodies, even in large numbers, are not a health hazard. There is an aesthetic and human problem rather than a direct threat of disease.
"[But] it will be extremely unpleasant in every sense. There will be bacterial decay over a number of days ... Identifying the dead will be an awful task, both for the relatives and the professionals. In many cases it will depend on things like dental records."
Geoff Rayner, the chairman of the UK Association of Public Health, said Ellis Island, the uninhabited former immigration centre off Manhattan, had large buildings that could be used as a makeshift morgue. "They are used to dead bodies in New York but I don't know where they begin with this," he said. "It's massive, it's overwhelming."Reuse content