The Internal Revenue Service warned Monday of a “frustrating” tax filing season in the coming weeks as the agency contends with a backlog of returns, staffing shortages and a massive increase in phone calls from taxpayers.

The IRS cautioned it is still working on “prior-year individual tax returns that have not been fully processed.”

“In many areas, we are unable to deliver the amount of service and enforcement that our taxpayers and tax system deserves and needs. This is frustrating for taxpayers, for IRS employees and for me,” IRS Commissioner Chuck Rettig said in a statement. “IRS employees want to do more, and we will continue in 2022 to do everything possible with the resources available to us.”