The Federal Aviation Administration issued a ground stop at Newark, LaGuardia and JFK International Airports Monday morning due to an equipment outage, according to an agency advisory.

Flights into Philadelphia International Airport and Teterboro Airport were also paused, the agency said. A spokesperson for the FAA said the ground stops at Newark, Philadelphia and Teterboro were issued due to issues with some frequencies at Philadelphia TRACON or Terminal Radar Approach Control— an air traffic control facility that guides aircrafts as they’re approaching and departing, typically covering a 30 to 50 mile radius. It’s unclear if outages at JFK and LGA were connected to the Philadelphia TRACON issue.

Some inbound and outbound flights appeared to resume at LGA and JFK as of 1:30 p.m., according to FlightAware's flight tracker. At Newark, flights appeared to remain grounded.

As many as 681 flights have been impacted as of Monday afternoon, according to the tracking service FlightAware. It is unclear when passengers with departures from the affected airports can expect flights to resume.

This story is developing and may be updated.