Following a series of close calls involving commercial flights, aviation leaders met Wednesday to discuss the incidents and work to identify patterns and factors fueling risks to the industry.
At the safety summit in Washington, Secretary of Transportation Pete Buttigieg and acting Federal Aviation Administration Administrator Billy Nolen both acknowledged an "uptick" in close calls in recent weeks.
"Initial information suggests that more mistakes than usual are happening across the system on runways, at gates when planes are pushing back, in control towers and on flight decks," Buttigieg said in remarks at the event.
The FAA announced the summit last month as part of a review of the nation's aerospace system, looking at structure, culture, systems and integration of safety efforts.
Rich Santa, head of the national air traffic controller's union, said Wednesday there's a "staffing issue" among the air traffic controller workforce and there are 1,200 less certified professional controllers than there were 10 years ago.
"When you have less eyes, less positions open, that demands a reduction of efficiency to contest the safety risks that are introduced," Santa said.
ABC News is aware of at least six close calls involving commercial flights in recent months. Ahead of the summit, Buttigieg said in an interview with ABC News Transportation Correspondent Gio Benitez that officials are on track to record "more than 20" close calls with aircraft this year.
Nolen on Wednesday called such incidents "concerning."
"There is no question that aviation is amazingly safe, but vigilance can never take a day off," he said.
Still, officials have stressed the safety of the nation's aviation system -- citing the 45,000 flights that take off a day and the fact that there hasn't been a fatal commercial plane crash since 2009. (There has been a more recent deadly aviation incident, when a passenger was killed on a Southwest flight in 2018 after an engine failure.)
Data shows the more serious incidents involving aircraft close calls have been declining over the last two decades even as the total number of incidents grows.
National Transportation Safety Board Chairwoman Jennifer Homendy said Wednesday while those stats are true, "the absence of a fatality or an accident doesn't mean the presence of safety."
Homendy also stressed the amount of safety recommendations her agency has made that have not been acted on by regulators.
"How many times are we going to have to issue the same recommendation over and over and over again?" she said.
Buttigieg told ABC News before Sunday that the rebound of aviation from lows seen during the COVID-19 pandemic has been "putting strain on the system" and the increased demand in air travel also may be contributing to the recent close calls.
Aviation leaders stressed that point at Wednesday's summit
"The industry is ramping back up from the pandemic, during which a number of people ... were either retired or were laid off layoff," Homendy said. "A new workforce is coming in that needs to be appropriately, adequately trained. Some who were out during the pandemic also need to be retrained."
When asked in the ABC News interview if experience among aviation workers is to blame, Buttigieg pushed back, saying, "It's not just experience in general."
"We are still talking about human beings," he told ABC News. "And while, again, these cases are extremely rare -- we're not going to allow any of them to pass by without getting a very close, focused look at how it happened, why it happened and what steps could prevent it from happening."
Buttigieg wrote to attendees of the safety summit on Tuesday, saying he expected to accomplish three things: identifying "patterns and risk factors," identifying how the different parts of the aviation system can "address any risks" and identifying and implementing "additional steps" to reduce those risks.
"It is our responsibility to take a hard look at all factors and determine what steps are needed to reinforce a safety culture and strengthen safety practices, especially given significant disruptions and changes to the aviation sector coming out of a global pandemic," Buttigieg wrote.
FAA and NTSB investigating a string of incidents
The FAA and NTSB are currently investigating six close calls involving commercial planes in recent months -- five of which occurred at airports and one over the Pacific Ocean.
In December, United Flight 1722 bound for San Francisco experienced a sudden loss in altitude over the Pacific, officials have said. Shortly after takeoff from Maui, the Boeing 777 dropped to just 775 feet above the water in less than 20 seconds. The plane was then able to regain altitude and continue on to its final destination. There were no injuries.
On Jan. 13, an American Airlines flight crossed a runway at New York City's John F. Kennedy International Airport without clearance from air traffic control, causing a Delta Air Lines plane to abort its takeoff from that runway, according to the NTSB. The closest point between the two aircraft was about 1,400 feet, a preliminary report from the agency stated.
Also in January, United Airlines 384 crossed a runway at Honolulu's airport without clearance from air traffic control. A Cessna was landing on the same runway at the time. The Cessna came to a stop approximately 1,170 feet from the United flight, according to the FAA. There were no injuries or damage to the aircraft.
On Feb. 4, a FedEx plane landing at Austin-Bergstrom International Airport in Texas came within 100 feet of a Southwest Airlines jet taking off on the same runway, the FAA said. Both planes were given clearance from air traffic control to use that runway.
And in February, two more close calls, according to officials -- one of which occurred at Sarasota-Bradenton International Airport in Florida after an air traffic controller cleared an American Airlines flight to land on the same runway an Air Canada Rouge plane was cleared to take off from. The American flight self-initiated a go-around and the two aircraft were approximately 3,100 feet from each other.
The other incident occurred at Boston Logan International Airport after a Learjet took off without clearance from air traffic control as a JetBlue flight was preparing to land on an intersecting runway.
ABC News' Clara McMichael contributed to this report.