Flights are delayed at LaGuardia airport in New York City because of “staffing issues” among air traffic controllers due to the government shutdown.

Newark Liberty Airport in New Jersey and Philadelphia International Airport were also experiencing delays Friday morning, although the reasons why weren't immediately clear. The Federal Aviation Administration only cited low staff for the problems at LaGuardia, but the air traffic controllers' union executive vice president blamed all the delays on the government shutdown, according to CNN.

LaGuardia and Newark Liberty were operating with delays of an hour or more, according to the FAA’s status website. Philadelphia International Airport is also still experiencing delays of up to 15 minutes, but the official warning has been removed.

The Federal Aviation Association had briefly halted flights entirely at LaGuardia, one of the busiest airports in the United States, Friday morning, although that notice has now been lifted.

Since the government shutdown began on Dec. 22, air traffic controllers and security agents have been working without pay, and many have called in sick. On Wednesday, unions for aviation employees warned that the prolonged shutdown could disrupt passenger safety procedures.

According to the FAA, more air traffic controllers than usual called in sick in Washington, D.C., and Florida on Friday, impacting airports along the East Coast. Even prior to the shutdown, staffing in air-traffic control facilities was at a 30-year low.

The union representing aviation mechanics and other FAA employees also began a strike at Cleveland Hopkins International Airport on Friday, according to News 5 Cleveland. It’s illegal for most federal employees to strike. But on Monday, the president of the Association of Flight Attendants, Sara Nelson, advocated for private sector workers to enact a general strike in solidarity with those unable to protest in the public sector.

“We have a growing concern for the safety and security of our members, our airlines, and the traveling public due to the government shutdown,” the National Air Traffic Controllers Association, Air Line Pilots Association, and Association of Flight Attendants said in a joint statement Wednesday. “This is already the longest government shutdown in the history of the United States, and there is no end in sight. In our risk-averse industry, we cannot even calculate the level of risk currently at play nor predict the point at which the entire system will break. It is unprecedented.”

Newly elected Democratic Rep. Alexandria Ocasio-Cortez had warned about the impact of the shutdown on air traffic control in her first speech on the House floor — as she reminded Twitter on Friday. LaGuardia Airport is within her district.

“Do you understand the economic + social impact of grounding flights because two people are insisting on a project *now* that the majority of Americans oppose?” Ocasio-Cortez tweeted, referring to the budget impasse between Congress and President Donald Trump over funding a $5.7 billion border wall, which catalyzed the shutdown. On Sunday, 3,000 screeners for the Transportation Security Administration called out of work. Days later on Friday, many of the 800,000 federal employees impacted by the partial government shutdown missed their second paycheck.