More than 700 flights have been canceled and around 4,000 delayed across the United States after an equipment outage disrupted operations at Newark Liberty International Airport in New Jersey and Philadelphia International Airport in Pennsylvania.

Axar.az reports, citing ABC News, that the disruption began Monday morning after a circuit failed at a Philadelphia air traffic control facility.

When the backup system was activated, officials discovered that a backup line in New Jersey had also been cut. The FAA temporarily expanded ground stops to New York’s John F. Kennedy and LaGuardia airports while crews worked to replace the circuit.

Operations at JFK and LaGuardia have since resumed, but flights remain halted at Newark and Philadelphia, with no estimated time for a full resumption.

The outage has caused widespread disruptions across the Northeast, with flights diverted, delayed or canceled.

United Airlines said the “FAA technology outage is affecting all airlines in the Northeast,” adding that flights bound for Newark had been held at departure airports or diverted to other airports.