Julia Horowitz

(CNN Money) -- It's tax day — and the IRS is experiencing technical difficulties.

The service that taxpayers use to file online is partly down, according to Acting IRS Commissioner David Kautter.

"A number of IRS systems are unavailable at the moment," Kautter said at a Congressional hearing Tuesday morning. "We are working to resolve this issue and taxpayers should continue to file their returns as they normally would."

In a statement, the IRS confirmed that the problems are ongoing.

The agency did not provide details on the nature or extent of the technical issues.

Kautter told members of Congress that the problems appear to be related to the "transmission" of tax returns from software providers like H&R Block and Intuit, which runs TurboTax.

H&R Block said that despite the outage, it's still processing tax returns for its customers.

"While the IRS system is down, we are completing the returns, which will be sent as soon as the IRS system re-opens and will be considered filed on time," the company said in a statement. "We are encouraging tax payers to continue to use our retail services or our do-it-yourself products as they normally would."

Intuit had similar advice for those using TurboTax.

"Taxpayers should go ahead and continue to prepare and file their taxes as normal with TurboTax," a company spokeswoman said. "TurboTax has uninterrupted service and is available and accepting e-filed returns. We will hold returns until the IRS is ready to begin accepting them again."

Tuesday is the last day for taxpayers to file their tax returns, unless they've filed for an automatic six-month extension. But an extension to file is not an extension to pay. Filers must pay any additional money they owe to Uncle Sam for 2017 today.

Related: It's tax day, and you're scrambling. What's the penalty for filing taxes late?

Lawmakers are calling on the IRS to avoid penalizing those affected by the glitches.

"Tax Day is already a stressful time for millions of Americans, even when everything goes right," Richard Neal, the ranking Democrat on the House Ways and Means Committee, said in a statement. "Given this news, I hope that the IRS will make accommodations so that every taxpayer attempting to file today has a fair shot to do so without penalty."

— CNN's Gregory Wallace and Jeanne Sahadi contributed reporting.