U.S. Treasury Secretary Jacob J. Lew said U.S. borrowing authority may not last past Feb. 27 and urged Congress to extend the debt ceiling as soon as possible.

Extraordinary measures used by the Treasury to keep under the debt ceiling “are likely to be exhausted in less than three weeks,” Lew said today in a letter to House Speaker John Boehner, a Republican from Ohio.

“Congress is scheduled to be out of session for part of that time, and it would be a mistake to wait until the last possible minute to act,” he said.

House Republican leaders say they want concessions in exchange for raising the debt limit, though they’ve been unable to agree on exactly what. President Barack Obama and Senate Majority Leader Harry Reid of Nevada have refused to negotiate on the issue.

Treasury on Friday employed its first extraordinary measure, suspending its state and local series of non-marketable securities.

After the extraordinary measures run out, the Treasury will be left with about $50 billion in cash, Lew said.

“That number, however, could be materially higher or lower, depending on the pace of tax refund filings,” Lew said.

“Regardless, any foreseeable cash balance would be exhausted quickly. In previous years, the Internal Revenue Service has issued as much as $10-15 billion in refunds on a single day and nearly $40 billion in a single week.”