US telecom networks took a hit from superstorm Sandy, which knocked out some emergency call centers in the northeast, the Federal Communications Commission said Tuesday.

FCC chairman Julius Genachowski told reporters that a "very small number" of emergency call centers were knocked out in the storm, causing urgent 911 calls to be rerouted, in some cases without the normal location identifiers.

"We take these issues very seriously and are looking into all reports," Genachowski said, without providing further details.

Overall, Genachowski said Sandy delivered a "substantial and serious" hit to the telecom infrastructure in the northeastern United States.

"Our assessment is that communications outages could get worse before they get better, particularly for mobile," he said, adding that flooding and snow in some areas could hamper recovery efforts.

FCC officials said that in the "core" region -- identified as 158 counties in 10 states from Virginia to Massachusetts -- around 25 percent of wireless cell transmission sites were out of service.

Officials said it was not possible to tell how many customers had no wireless telephone service, because they may be able to access using nearby cells. Fixed line telephone service suffered less damage, officials said.

"Information is still coming in and it changes from hour to hour," said David Turetsky, the FCC's head of public safety and homeland security.

He added that some wireless cells had switched to backup electric power after outages, but that "many sites were running out of backup power."

The FCC said 25 percent of people in the affected areas also lost access to cable television or cable Internet service.

The major wireless carriers said they had outages and were working to restore service.

Verizon said 94 percent of its cell sites were "up and running, and all our switching and data centers are functioning normally," but that some service was disrupted by power outages and flooding in low lying areas such as lower Manhattan.

AT&T said it was experiencing "some issues in areas heavily impacted by the storm," and that teams would be deployed to restore service "as soon as it is safe to do so."

Sprint reported "impacts in the states affected" due to loss of commercial power, flooding and other issues but offered no numbers. "Our technicians are assessing the damage and servicing sites as they become known to us and as the areas are deemed safe to enter," a Sprint statement said.

T-Mobile also said some service was disrupted, without elaborating. "T-Mobile rapid response engineering teams are assessing the situation and we are moving as quickly as possible," the carrier said.