A $600 (£459) jobless benefit paid weekly to tens of millions of unemployed Americans expired overnight after Republicans and Democrats failed to reach a deal on its extension.
Friday was the deadline for an agreement on the coronavirus relief, which tops up state benefits that are mostly far below $600.
Republicans want a smaller relief plan than one passed by the Democrat-controlled House of Representatives.
Talks will continue on Saturday.
Unemployment has soared during the coronavirus pandemic, with 30 million Americans currently collecting jobless benefits.
The US economy shrank at a 32.9% annual rate between April and June – the sharpest decline for decades.
The $600 payments are part of a $2.2tn congressional package adopted in March as coronavirus forced businesses to shut down.
But Congress did not anticipate the current resurgence of the virus, deepening the impact on the economy.
The Covid-19 pandemic has killed at least 152,384 people in the US – the highest death toll in the world.