(CNN) Treasury Secretary Steven Mnuchin on Friday issued a warning Friday that the US government is at risk of running out of cash sooner than expected.

In a letter to Democratic House Speaker Nancy Pelosi, Mnuchin wrote that the US might default on its obligations as soon as early September, before Congress returns from its summer recess

"Based on updated projections, there is a scenario in which we run of out cash in early September, before Congress reconvenes," Mnuchin wrote in a letter.

The federal government has not been able to borrow money since March, when congressionally-mandated borrowing limits went back into force. The US Treasury, which is facing a growing deficit thanks in part to President Donald Trump's 2017 tax cuts, had said it had enough cash to last until the fall.

Pelosi and Mnuchin have spoken three times this week in an effort to jumpstart stalled budget and debt ceiling negotiations, and multiple sources say the new default projections have provided an accelerant for those efforts.

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