WASHINGTON (AP) — Treasury Secretary Janet Yellen is telling Congress that the Treasury Department will likely exhaust all of its "extraordinary measures" to avoid an unprecedented default on the government's obligations by Oct. 18.

Yellen warned Congress three weeks ago that the Treasury would run out of maneuvering room by mid-October. In a Tuesday letter, she said economists were able to issue a specific date based on the amount of revenue the U.S. had received in September from corporate and private quarterly tax payments.

Yellen's letter arrived the day after Senate Republicans blocked consideration of a bill to deal with the debt limit and approve a stopgap funding bill to avert a government shutdown Friday.

"We now expect that Treasury is likely to exhaust its extraordinary measures if Congress has not acted to raise or suspend the debt limit by Oct. 18," Yellen said. "At that point, we expect Treasury would be left with very limited resources that would be depleted quickly."

Yellen said it was uncertain whether Treasury could meet all of the nation's commitments after that date.