(CNN) — The Federal Aviation Administration delayed flights into New York and Newark, New Jersey, and it briefly halted flights bound for Philadelphia International Airport and New York’s LaGuardia airport, as smoke from Canadian wildfires continues to spread across the northeastern United States.

The so called ground stop required planes scheduled to fly to Philadelphia and LaGuardia to remain on the ground at their airport of origin. The LaGuardia ground stop was lifted at 7:45 am ET, and downgraded to a “ground delay.” The Philadelphia ground stop expired at 9:15 am ET.

The FAA also is delaying flights bound for Newark Liberty International Airport. The visibility at Newark at 9 am ET was reported as three miles. The FAA’s ground delay program is currently set to expire at 9:59 am ET.

An FAA advisory says flights nationwide are delayed 34 minutes on average with the maximum delay lasting one hour 47 minutes.

Delays are expected into and out of LaGuardia until late Thursday night. The FAA’s operations plan for Thursday shows smoke potentially impacting travel from New York to Charlotte throughout the day.

“The FAA will likely need to take steps to manage the flow of traffic safely into New York City, DC, Philadelphia and Charlotte due to reduced visibility from wildfire smoke,” said a statement from the agency.

Meanwhile, United Airlines says it is isuing a travel waiver for passengers impacted by the unprecedented haze that is snarling flights on the East Coast. United is waiving change fees and fare differences on Thursday and Friday for customers with flights passing through 12 different airports, including its major hubs at Newark and Washington Dulles International Airport.

“We are monitoring the situation closely and looking out for the safety of our employees who work outside and adjusting our schedule where needed,” United said in a statement.

More than 1,100 flights to, from and within the United States have been delayed, with 71 of them canceled, according to tracking service FlightAware. New York’s JFK airport and Newark Liberty Airport in Newark, New Jersey, were the hardest hit by the haze. JFK had two canceled flights and 44 delays, representing 6% of its schedule. Newark had three canceled flights and 52 delays, representing 8% of its flights. LaGuardia had 63 delays and three cancellations, making up 11% of its scheduled flights. Wednesday ended with 5,600 US flight delays and 162 cancellations, although not all were due to the smoke.

US Secretary of Transportation Pete Buttigieg tweeted Wednesday that, “Smoke from Canada’s wildfires is affecting visibility in our airspace and leading to delays. The FAA is fully prepared to modify operations as needed.”

On Wednesday, the FAA issued a similar ground stop for LaGuardia airport.

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