Flights have been stopped at Newark Liberty International Airport in New Jersey and Philadelphia International Airport in Pennsylvania due to equipment outages, according to the Federal Aviation Administration, with no estimated time for resuming.

More than 400 flights have been canceled nationwide, with over 4,000 delays reported on Monday. Newark was hit the hardest with over 280 cancellations.

FAA Administrator Bryan Bedford said a circuit blew at a Philadelphia air traffic control facility on Monday morning, and when the backup system went into place, officials discovered the backup line had been cut at a New Jersey construction site.

In order to change the circuit in Philadelphia, Bedford said the airspace in the Northeast would be closing for at least 30 minutes, and ground stops were expanded to include New York City’s John F. Kennedy International Airport and LaGuardia Airport as part of the work.

Airspace has since reopened at JFK and LaGuardia, but officials said the fix to replace the circuit in Philadelphia has not worked, leaving operations still halted at Philadelphia and Newark’s airports, with no time estimate for when they’ll resume.

Some inbound flights have been diverted to other airports.

United Airlines, which has one of its hubs at Newark Airport, said the “FAA technology outage is affecting all airlines in the Northeast.”

“Flights that had been destined for Newark have either been held at their departure airport or diverted to alternative airports,” United said in a statement. “Once the FAA outage is resolved, we will work to safely get our customers to their destinations, and have issued a travel waiver to give our customers flexibility to manage their travel plans.”

This is a developing story. Please check back for updates.

ABC News’ Jason Volack contributed to this report.