WASHINGTON, Nov 9 (Reuters) - Major airlines braced for

a third day of government-mandated flight cuts Sunday after

rising air traffic control staffing shortages snarled thousands

of flights on Saturday.

The Federal Aviation Administration instructed airlines to

cut 4% of daily flights starting on Friday at 40 major airports

because of air traffic control safety concerns. The shutdown,

which has reached a record 40 days, has led to shortages of air

traffic controllers who, like other federal employees, have not

been paid for weeks.

Reductions in flights are mandated to reach to 6% on Tuesday

and then hit 10% by November 14.

The FAA said on Saturday there were air traffic control

staffing shortages impacting 42 airport towers and other centers

and delaying flights in at least 12 major U.S. cities including

Atlanta, Newark, San Francisco, Chicago and New York.

Some 1,550 flights were canceled and 6,700 flights were

delayed on Saturday, compared with Friday when 1,025 were

canceled and 7,000 were delayed.

Airline officials privately said the number of delay

programs made it nearly impossible to schedule and plan many

flights and expressed alarm about how the system would function

if staffing issues worsen.

The cuts, which began on Friday morning, include about 700

flights from the four largest carriers: American Airlines ( AAL )

, Delta Air Lines ( DAL ), Southwest Airlines ( LUV ) and

United Airlines. The airlines are due to cancel about

the same number of flights Sunday.

During the government shutdown, 13,000 air traffic

controllers and 50,000 security screeners have been forced to

work without pay.

U.S. Transportation Secretary Sean Duffy said it was

possible he could require 20% cuts in air traffic if more

controllers stop showing up for work. "I assess the data," Duffy

said. "We're going to make decisions based on what we see in the

airspace."

Republican Senator Ted Cruz said he was told by the FAA that

since the shutdown started pilots have filed more than 500

safety reports about mistakes made by air traffic controllers

because of fatigue.