United Airlines' top boss warned that the carrier has to change or even cut back on its schedule at one of its main hubs as air traffic controller staffing shortages persist. 

CEO Scott Kirby told staff in a note Saturday reviewed by FOX Business that Port Authority is helping the carrier get more gates at New Jersey's Newark Liberty International Airport, which he said "are critical to avoid gridlock on the taxiways." 

Even still, Kirby said the carrier is "going to have to further change/reduce our schedule to give ourselves even more spare gates and buffer - especially during thunderstorm season."   

Newark is "the best international gateway that exists anywhere in the country. But it is also "the most operationally difficult airport in the country," Kirby said.  

Kirby's remarks come after carriers faced significant amount of disruptions last week due to thunderstorms that rolled into the East Coast and the Midwest. However, Kirby noted that Federal Aviation Administration (FAA) staffing issues exacerbated problems, leading to tens of thousands of flight disruptions in the New York area.  

While there were flight disruptions at all the major New York City-area airports, United faced "severe restrictions on flight operations" at Newark. 

Over 67% of flights were delayed and nearly 30% were canceled at Newark on Sunday, according to data from flight tracking website FlightAware. On Monday, nearly 50% were canceled and over 72% were delayed at the airport. 

Kirby even admitted that it "one of the most operationally challenging weeks I've experiences in my entire career."

When the disruptions first began, Kirby said in a note that the reduced arrival and departure rates were "almost certainly a reflection of understaffing/lower experience at the FAA." 

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On June 24, arrival rates plummeted 40% at Newark and departure rates dropped by 75%. 

However, Kirby said he had "constructive conversations" with the FAA and Transportation Secretary Pete Buttigieg since the disruptions upended operations. 

"The current FAA leadership inherited these challenges and to their credit, they have been public that they're thousands of controllers short – and the NY/NJ airspace, in particular, is understaffed," he said. 

Kirby added that N90, which manages all the New York and New Jersey area airports, "is also probably the most technically challenging job anywhere in the world in aviation which means that experience also makes a difference." 

Recently, however, Buttigieg said on CBS's "Face the Nation that "severe weather was the key cause of recurring clusters of flight problems for thousands since January.

FOX Business' Timothy Nerozzi contributed to the report.