U.S. Transportation Secretary Sean Duffy and Republican leaders in Congress warned Thursday that flight disruptions may increase as a government shutdown enters its 23rd day and controllers miss their first full paycheck.

Some 13,000 air traffic controllers and about 50,000 Transportation Security Administration officers must work without pay during the government shutdown. Controllers will miss their first full paycheck on Tuesday.

Duffy said last week air traffic control absences account for 53% of flight delays since the shutdown began, compared with 5% normally.