Janet Yellen warns of a possible October default of U.S. debt, swollen by the pandemic.

The United States could default on its debt sometime in October if Congress does not take action to raise or suspend the debt limit, Treasury Secretary Janet L. Yellen warned on Wednesday.

The “extraordinary measures” that the Treasury Department has been employing to finance the government on a temporary basis since Aug. 1 will be exhausted next month, Ms. Yellen said in a letter to lawmakers. She added that the exact timing remained unclear but that time to avert an economic catastrophe was running out.