Rescue workers have been ordered to pull back from two buildings in the World Trade Center complex, which officials say are in danger of collapsing.
The buildings, 1 Liberty Plaza and the Amex Building were both damaged in Tuesday's attack.
The news came after five New York City fire fighters were found alive under the rubble.
Television reports say they had been communicating with colleagues during the day. Two men were able to walk out of the debris while the three others were reported to be in good condition.
More than 5,000 people were declared dead or missing in preliminary tolls after the terror strikes on New York and Washington on Tuesday.
New York's mayor Mr Rudy Giuliani told a mid-morning news conference that 4,763 people were missing in the still smoking ruins of the World Trade Center which collapsed 48 hours earlier soon after being hit by two hijacked airliners.
The missing included the 157 passengers and crew known to have been on board the planes, which exploded in huge fireballs that spewed out of upper floors of the 110-storey twin towers, Mr Giuliani said.
He said 94 bodies had been recovered so far, and that 46 of them had been identified.
In Washington, the Pentagon said "initial estimates indicate that approximately 126 persons remain unaccounted for in the wake of Tuesday's attack" on the heart of the US defence establishment.
"These numbers do not include any of the passengers aboard American Airlines Flight Number 77," it said in a statement.
The airline reported earlier that the plane was carrying 64 passengers and crew when it slammed into the building.
The statement said those unaccounted for included 68 army personnel, including 47 civilians; six army contractors; 42 navy personnel, including nine civilians; and 10 defence agency personnel.
In addition to entire bodies extracted from the ruins of the World Trade Center, "we also have the gruesome, horrible situation where we recover body parts, and there are 70 people in that category. There are 70 body parts," Mr Giuliani said.
"I am sorry I have to describe it that way," he added.
"They will be numbered for identification" by forensic examiners, he added.
Mr Giuliani said about 6,000 tonnes of debris had been removed from the disaster area.
But that was only a fraction of the mountain of shattered glass, concrete and twisted metal that had once been the pride of New York, and he estimated that it would take between two and three weeks to clear the site.
AFP