WASHINGTON (Reuters) – U.S. Treasury Secretary Janet Yellen on Tuesday again warned lawmakers that the nation was close to exhausting its federal borrowing capabilities – now set for Oct. 18 – and urged Congress to act in order to avert “serious harm” to the economy.

In her most recent letter to Congress, she stated that “at that point we expect Treasury will be left with very few resources that would be exhausted quickly.”

“It is uncertain whether we could continue to meet all the nation’s commitments after that date,” she added in the letter, one day after Senate Republicans rejected a measure to raise the nation’s borrowing limit to pay for previously incurred government spending.

To keep the government’s funds flowing, the Treasury already took “extraordinary steps” after the debt ceiling had been reached in the summer. However, these measures will expire within 20 days. But the precise date can vary according to Yellen.

This uncertainty “underlines the crucial importance of not waiting until the debt limit is raised or suspended to increase it,” she stated.