Crews are removing pieces of the broken crane that killed four construction workers and damaged several buildings.
Mayor Michael Bloomberg said that two other workers and a woman who was visiting the townhouse remain missing. The mayor says rescuers will be able to intensify a search for survivors once the large pieces of debris are removed.
The accident happened on 51st Street near 2nd Avenue at about 2.20pm (7.20pm GMT) at the construction site of a block of flats.
The crane toppled over splitting into pieces as it fell pulverising a four-story townhouse and demolishing parts of three other buildings. One man was pulled from the townhouse three and a half hours after the building was crushed.
About 19 of the 44 storeys of the crane had been erected and it was to be extended today when a piece of steel fell and sheared off one of the ties holding it to the building, said Stephen Kaplan, an owner of Reliance Construction Group, which manages construction at the
Mr Kaplan said the company had subcontracted the work to different companies and was not in charge of the crane. He was not sure if any workers at the site were among those killed.
The collapse devastated the block on Manhattan's East Side, cars were overturned and crushed. A huge dust cloud rose over the neighbourhood. Rubble was piled several stories high.
An intensive rescue operation was under way to find those trapped in the rubble on 51st Street near 2nd Avenue.