The agency stopped accepting electronically filed tax returns because of the problem. The outage could affect refunds, but the agency said it doesn't anticipate "major disruptions."
"The IRS is still assessing the scope of the outage," the agency said. "At this time, the IRS does not anticipate major refund disruptions; we continue to expect that nine out of 10 taxpayers will receive their refunds within 21 days."
The IRS.gov website remains available, but "Where's My Refund" and other services are not working.
Some systems will be out of service at least until Thursday, the agency said. "The IRS is currently in the process of making repairs and working to restore normal operations as soon as possible," the IRS said.
Taxpayers can continue to send electronic returns to companies that serve as middlemen between taxpayers and the IRS. But those companies have to hold on to the tax returns until the IRS systems are up and running again, the IRS said.
People who have already filed returns don't need to do anything more, the IRS said.
- Melanie Eversley, USA TODAY 7:39 a.m. EST February 4, 2016