NEW YORK, July 3 (Reuters) - New York City will continue searching for remains of victims of the Sept. 11 attacks indefinitely, saying on Tuesday it could no longer complete the task in one year as planned.

Bits of human remains continue to turn up more than five years after the attacks that killed 2,759 people in New York, including the 10 hijackers aboard the two jetliners that crashed into the World Trade Center.

Remains have not been found for all of the victims. Surviving relatives say it helps with their grieving to know that some physical remains have been recovered.

Edward Skyler, New York's deputy mayor for administration, told Mayor Michael Bloomberg in a report on Tuesday that the city medical examiner's office will continue to sift through excavations in areas surrounding the crash site, looking for human remains.

"Due to the additional material that needs to be excavated and sifted, the one-year goal for this operation is no longer attainable," Skyler wrote. "In fact, our experience over the last nine months and the ongoing rebuilding of the World Trade Center will continue in one form or another for the foreseeable future."

The former World Trade Center site and surrounding areas are now a construction zone, with four skyscrapers in the works. In addition, a former bank building just off the site where some remains have been found is being demolished to make way for a new office tower.

In all 2,992 people died in the 2001 attacks in New York, Virginia and Pennsylvania.

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