The U.S. government could run out of resources to meet the nation’s obligations as soon as Dec. 15, Treasury Secretary Janet Yellen said Tuesday, reviving questions about how Congress will resolve a standoff about raising or suspending the federal borrowing limit.

Ms. Yellen provided the new estimate of when the federal government might no longer be able to pay all of its bills in a letter to Congressional leaders. She had previously said that a debt-limit increase passed by Congress in October provided confidence that the...

The U.S. government could run out of resources to meet the nation’s obligations as soon as Dec. 15, Treasury Secretary Janet Yellen said Tuesday, reviving questions about how Congress will resolve a standoff about raising or suspending the federal borrowing limit.

Ms. Yellen provided the new estimate of when the federal government might no longer be able to pay all of its bills in a letter to Congressional leaders. She had previously said that a debt-limit increase passed by Congress in October provided confidence that the federal government would be able to pay its bills at least through Dec. 3.

“To ensure the full faith and credit of the United States, it is critical that Congress raise or suspend the debt limit as soon as possible,” she wrote.

A transfer tied to President Biden’s signing Monday of a $1 trillion infrastructure bill could leave the Treasury short on cash. The law instructs the Treasury to move $118 billion to the Highway Trust Fund, which Ms. Yellen said would happen Dec. 15.

“While I have a high degree of confidence that Treasury will be able to finance the U.S. government through December 15 and complete the Highway Trust Fund investment, there are scenarios in which Treasury would be left with insufficient remaining resources to continue to finance the operations of the U.S. government beyond this date,” she wrote.

The letter doesn’t indicate that Dec. 15 is the definite date when the U.S. may need to forgo certain obligations. Private analysts have said that date could fall between mid-December and mid-February.

As lawmakers prepare for another hike in the debt ceiling, WSJ's Greg Ip explains why it’s economically feasible for the U.S. to keep borrowing, as long as interest rates stay low. The Wall Street Journal Interactive Edition

The Treasury Department has been using what it calls extraordinary measures, such as suspending certain investments, to conserve cash as it bumps against the $28.88 trillion borrowing limit set by Congress. Raising the debt limit doesn’t authorize new spending, but instead allows the government to issue new debt to pay for existing obligations, such as Social Security benefits and interest on the debt.

If the debt ceiling isn’t addressed and the Treasury’s available funds dwindle, the department could be forced to suspend certain payments or place a higher priority on the timing of some over others. Such a move could affect families, older Americans and others who rely on regular payments from the federal government, administration officials have warned.

Wall Street firms, economists and administration officials have also warned that if the Treasury were forced to delay interest payments on the debt, which would constitute default, it would roil financial markets and possibly trigger a recession.

Even just approaching the so-called X-date, or the date when the government will no longer be able to honor all its obligations fully, can have serious consequences, analysts have said. In 2011, Standard & Poor’s stripped the U.S. of its triple-A credit rating for the first time after the Treasury came within days of being unable to pay certain benefits.

The path to addressing the debt limit in Congress is poised to be a reprise of the partisan fight waged earlier this year. Lawmakers in October were able to pass a measure lifting the debt ceiling by $480 billion after Senate Minority Leader Mitch McConnell (R., Ky.) offered to raise the limit on a short-term basis, gathering 11 GOP votes to join Democrats and break a filibuster of the bill. Some Republicans, including Sen. John Cornyn (R., Texas.), said that the GOP wouldn’t again provide votes to resolve the issue. “I think it was a one time event,” he said.

Mr. McConnell on Tuesday didn’t say whether Republicans would again seek to force Democrats to use a legislative process called reconciliation to raise the debt ceiling without GOP cooperation.

“We’ll figure out how to avoid default, we always do,” Mr. McConnell said.

Democrats refused to use reconciliation to raise the debt limit in October because they said both parties had historically cooperated in addressing the issue and warned that the process was time-consuming and could risk default. Democrats again said that Republicans should support efforts to raise the debt limit, but some didn’t rule out using reconciliation to address the issue this time around.

The date when the government will no longer be able to fully pay all its bills is difficult to predict because some of the federal government’s expenditures and receipts can vary widely from day to day, said Shai Akabas, director of economic policy at the Bipartisan Policy Center, a think tank. Mr. Akabas said that variance had been heightened during the Covid-19 pandemic because of government spending on new aid programs.

The Bipartisan Policy Center in late October said the date would likely fall between mid-December and mid-February. Other analysts have said the date could arrive as soon as late December or early January.

“We are just trying to give people a sense of when the X-date arrives and when things could go south in a hurry,” Mr. Akabas said. “The fact that our window extends into February does not mean that Congress should wait to act until February.”

Write to Amara Omeokwe at amara.omeokwe@wsj.com and Andrew Duehren at andrew.duehren@wsj.com