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The Internal Revenue Service is among the federal agencies affected by the government shutdown, and the possibility of a lengthy stalemate could complicate this year's tax season.

The IRS is currently working under non-filing season shutdown plans, but it's not clear how those might be updated one filing season hits. Right now, the agency is operating with only 12.5% of its workforce, or fewer than 10,000 federal employees.

During a shutdown, the IRS typically doesn't perform audits, pay refunds or offer assistance to taxpayers if they have questions — especially outside of the filing season.

And filing season is coming: Tax filing season usually begins in mid-January (the IRS has yet to announce when individuals and businesses can begin submitting their income tax returns this year).

For now, individuals who call the IRS with questions are greeted with an automated message: "Welcome to the IRS. Live telephone assistance is not available at this time. Normal operations will resume as soon as possible."