Flights were taking off and landing at New York City-area airports Tuesday with some delays and cancellations, after hours of disruptions resulting from damaged communications lines at a key air traffic control facility in Philadelphia that were later repaired.

Hundreds of flights were canceled throughout the U.S. Northeast on Monday, including more than 600 into and out of Newark Liberty International Airport, as the outage hit every major airport in the New York City area. Transportation Secretary Sean Duffy also said in an online post that “it will take some time to clear all existing delays” despite the repairs.

The Newark airport showed about 40 delays and nearly 60 cancellations early Tuesday, according to Flightaware.com, a global flight tracking and aviation data site.

The primary communications line at the Federal Aviation Administration facility in Philadelphia had failed earlier Monday, while the backup fiber optic line was inadvertently severed by railroad construction workers. The failures left controllers without radar or communications for hours.

The outage brought commercial air traffic to a near-standstill at Newark for much of the day, with no commercial flights landing at the airport from about 9:40 a.m. to just after 5:20 p.m. local time, according to flight-tracking site Flightradar24.

More than 200 flights into and out of Philadelphia International Airport were also canceled, while disruptions hit LaGuardia and Kennedy airports in New York. Flights had resumed by Monday evening.

Some airlines were allowing customers to change flights without charge. The FAA advised travelers to check the status of their flights before heading to the airport.

The outage occurred just as President Donald Trump and numerous foreign leaders were flying into New York City for the U.N. General Assembly. Canada’s Prime Minister Mark Carney missed an event Monday on the sidelines of the General Assembly because of the flight disruptions, but his office said he had arrived in New York by Monday night.

The technical problems stem from the same FAA facility in Philadelphia that caused numerous disruptions at the Newark airport last spring.

Regional control centers like these have two lines that carry radar and communications data in and out: the primary and the backup. The FAA said a circuit on the primary failed Monday morning, and when the system tried to switch to the backup, workers discovered that the fiber optic line had been cut.

Earlier in the day, Duffy said an Amtrak construction crew was to blame. Amtrak, however, pointed to a New Jersey Transit construction crew that was working in the area between Newark and New Brunswick, New Jersey.

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AP reporters Rio Yamat and Jessica Hill contributed to this report from Las Vegas.

Josh Funk, The Associated Press