WASHINGTON – Cell phone towers knocked off line by superstorm Sandy were being gradually restored to service, but fueling generators for relay sites in areas without electricity is difficult, the Federal Communications Commission said on Thursday.

Sandy knocked out about 25 percent of cell sites on the U.S. east coast this week, the FCC said.

About 19 percent were still offline as of Thursday morning, the FCC said. Cell sites can be towers or smaller facilities which relay wireless calls.

“Restoration efforts in the hardest hit areas – including New York and New Jersey – continues to be more difficult,” said David Turetsky, chief of the FCC Public Safety and Homeland Security Bureau.

“Replenishing fuel supplies for generators that are enabling communications networks to continue operating is a particularly critical challenge,” he said in a statement.

(Reporting By Diane Bartz; Editing by David Gregorio)