The scope of the system issue was not immediately clear, but the IRS called it a "hardware failure." The IRS has started repairs and expects that some systems may remain unavailable until Thursday.

Computer failures hit several U.S. Internal Revenue Service systems Wednesday in the midst of tax season, leaving the agency unable to accept some taxpayer returns.

Taxpayers can still work on their returns but cannot submit them until the IRS can accept them again. The agency still hopes to process 90 percent of refunds within three weeks of filing.

"At this time, the IRS does not anticipate major refund disruptions," the agency said in a statement.

Several tools, including a service that tracks refunds, are currently unavailable.

Most taxpayers must file their returns by April 18.