Major flight delays due to staffing cuts among US air traffic controllers continued on Tuesday and officials warned more was yet to come as the furloughs hit their third day.
US President Barack Obama's Republican foes accused his administration of "needlessly" disrupting thousands of passengers for political gain in the battle over mandatory budget cuts.
"As a result of the Administration's poor planning and, I would argue, political motives, thousands of people were stuck on tarmacs over the past few days," Senate minority leader Mitch McConnell said.
The Federal Aviation Administration said Tuesday it was working to try to "minimise" delays, but acknowledged that 1200 flights were delayed as a result of staffing shortages on Monday alone.
A further 1400 flights were delayed Monday because of the weather.
On Sunday, the first day of the staffing cuts, 400 flights were delayed due to the cuts. Tuesday's total was not yet tallied.
A trade organisation for the nation's airlines has filed a lawsuit seeking to halt the cuts, warning they could cause delays of up to four hours at major hub airports, affecting up to 6700 flights a day and one out of every three passengers.
The delays are expected to continue through October as the number of air traffic controllers on duty is cut by 10 per cent.
The FAA's 47,000 employees are being required to take unpaid days off as the agency seeks to slash its budget by five per cent.
"You would think that there would be lower-priority items for them to choose," Republican Senator John Thune said at a press conference.
The FAA said it is dealing with the staffing problems by having controllers "space planes farther apart so they can manage traffic with current staff."
Delays on Tuesday averaged 75 to 140 minutes at New York airports and 75 minutes in Washington while major hubs in Dallas, Los Angeles and Chicago were also affected. Poor weather contributed to the delays.
"As a result of employee furloughs due to sequestration, the FAA is implementing traffic management initiatives at airports and facilities around the country," the agency said in a statement.
"Travellers can expect to see a wide range of delays that will change throughout the day depending on staffing and weather-related issues."