"This was and still is a devastating storm with a serious impact on our nation's communications infrastructure," FCC Chairman Julius Genachowski said during a conference call reported by The Wall Street Journal.
Ongoing communication problems are expected as many of the towers still operating are having to depend on backup power, the FCC said.
"The storm is not over," Genachoswki said. "And our assumption is that communications outages could get worse before they get better, particularly for mobile networks, because of the flooding and loss of power."
A very small number of emergency call centers had lost power but calls were being rerouted, he said, urging people to only call 911 in life-threatening emergencies.
Keeping nonessential calls to a minimum and using texts, emails or social media to communicate would allow mobile networks to concentrate on priority calls, he said.
"We've seen broadband and social media continue to play an important role in communication for people during this storm," he said. "Social media is a critical platform for sharing information with loved ones. And it's been vital in keeping those other communications networks open for first responders."