The United States government shutdown entered its sixth day on Monday with no breakthrough in sight, as around two million federal workers went unpaid and some 750,000 were furloughed.

The White House has warned of mass layoffs of federal employees if the impasse continues. Press secretary Karoline Leavitt said the administration was moving ahead with workforce reductions, escalating the standoff with congressional Democrats.

Travel disruptions mounted, with more than 4,000 flights delayed on Monday as air traffic controllers worked without pay. Democrats are demanding an extension of Affordable Care Act subsidies as their price for reopening the government, while Republicans want a clean funding bill.

The Senate failed for a fifth time to advance a stopgap measure. With both sides entrenched, the shutdown is on track to become one of the longer ones in recent history, with economic effects rippling beyond Washington.

Economists warn that a prolonged shutdown could shave growth off the quarter’s GDP and delay key economic data releases. Past shutdowns have also dented consumer confidence, with effects lingering after the government reopens.