In New York, Mayor Rudolph Giuliani said yesterday evening he feared 10,000 people had been killed in the bombing which collapsed three buildings in the World Trade Center.
It could take weeks to dig through the rubble for victims.
"I have a sense it's a horrendous number of lives lost," Mr Giuliani said. "Right now we have to focus on saving as many lives as possible."
Hours after the attacks, huge clouds of smoke billowed from the ruins, obscuring much of the skyline.
Victims from the attack - many suffering from extensive burns - began arriving at hospitals in New York City about an hour after the two planes hit the twin towers of the World Trade Centre, witnesses said.
"Hundreds of people are burned from head to toe," said Dr Steven Stern at St Vincent's Hospital in Greenwich Village, lower Manhattan.
Doctors said the victims mostly had burns.
"So far we've received a few patients, mostly second-degree burns," Dr Gary Fishman at St Vincents said. "We are expecting the brunt of the people to arrive soon."
Most of the early patients were being sent to New York University's Downtown Hospital and to St Vincent's.
"The whole of lower Manhattan is coated in half an inch of dust," Reuters reporter Mr Daniel Sternoff said.
At St Vincents, hospital staff appealed for blood donors in the street, Reuters reporter Mr Ian Driscoll said. The line to give blood was over 100 people long.
"We expect smoke inhalation, trauma, and burns," Dr. Bernd Reisbeck said. "I expect we will be working non-stop for at least the next 24 hours."
AP