The number of air traffic controllers calling in sick since the U.S. government shutdown began last week has seen a slight "tick up," Transportation Secretary Sean Duffy said on Monday, cutting air traffic staffing by 50% in some areas at times, though he did not specify where the staffing issues were occurring.

Air traffic controllers and Transportation Security Administration officers must still turn up for work during the shutdown, but they are not being paid.

Duffy said there had been a "slight tick up in sick calls" in certain areas, noting that in some areas there was a 50% reduction in air traffic staffing on certain days, without indicating which airports were affected or for how long.