The Obama administration warned on Monday it could start defaulting on the government's obligations "very soon" after it runs out of room to borrow money under a legal cap on public debt.

The United States is due to reinstate a debt limit at the end of this week and Treasury Secretary Jack Lew said the administration can use accounting measures to stay under the new cap until the end of February.

Once the measures are exhausted, however, Lew said the government would burn through its remaining cash more quickly than it would at other times of the year because the Treasury will be issuing tax refunds.

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"Without borrowing authority, at some point very soon, it would not be possible to meet all of the obligations of the federal government," Lew said at a Bipartisan Policy Center event.

The debt ceiling has been a regular source of strife in Washington as Congress has argued over how to put the country's finances on a more stable path.

Lew said it was not clear that Washington needs to address the country's long-term fiscal problems this year because it has already made a lot of progress reducing the federal deficit.

"I'm not sure this is the year for the long-term fiscal challenge to be dealt with," he said.

"I actually believe that we've made so much progress in the short and medium term, we have a little time to deal with the longer term."

In October, Congress and the administration suspended a $16.7 trillion cap on borrowing until Feb. 7. If the debt ceiling isn't raised by then, Treasury can juggle money between government accounts for a few weeks to keep just under the new limit.

Once it loses the ability to borrow, Treasury would pay its bills by relying on incoming revenue and any cash left in public coffers.

After the money runs out, the government could start missing payments on its debt and other obligations, such as Social Security pensions. Many economists think a U.S. default could trigger a financial panic and perhaps even an economic depression.

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