WASHINGTON (Reuters) -U.S. airlines and travelers slogged through a second day of flight cuts across the country on Saturday as the government shutdown was expected to drive more cancellations in the days to come.  

The Federal Aviation Administration instructed airlines to cut 4% of flights on Saturday at 40 major airports because of the shutdown. The cuts will rise to 6% on Tuesday and then to 10% by November 14.

The cuts, which began at 6 a.m. ET (1100 GMT) on Friday, include about 700 flights from the four largest carriers - American Airlines, Delta Air Lines, Southwest Airlines and United Airlines.

Airlines will cut fewer flights on Saturday than Friday because of lower overall volume. United will cut 168 flights, down from 184 Friday, while Southwest will cancel just under 100 flights, down from 120.

During the record 39-day government shutdown, 13,000 air traffic controllers and 50,000 security screeners have been forced to work without pay, leading to increased absenteeism. Many air traffic controllers were notified on Thursday that they would receive no compensation for a second pay period next week.

The Trump administration has ramped up pressure on Congressional Democrats to agree to a Republican plan to fund the federal government, which would allow it to reopen. 

Raising the specter of dramatic air-travel disruptions is one such effort. Democrats contend Republicans are to blame for the shutdown because they refuse to negotiate over extending health insurance subsidies.

U.S. Transportation Secretary Sean Duffy said it was possible that he could require 20% cuts in air traffic if things get worse and more controllers do not show up for work.

"I assess the data," Duffy said. "We're going to make decisions based on what we see in the airspace."

Separate from the cancellations, absences of air traffic controllers on Friday forced the FAA to delay hundreds of flights at 10 airports including Atlanta, San Francisco, Houston, Phoenix, Washington, D.C., and Newark. More than 5,600 flights were delayed Friday.

Earlier this week, FAA Administrator Bryan Bedford said 20% to 40% of controllers were not showing up for work on any given day.