Comcast Corp. said Thursday evening that it would credit the accounts of customers who lost TV service during a partial national outage for subscribers using its new X1 set-top boxes.

The cable-TV giant apologized for the outage and confirmed that customers encountered delays when attempting to contact its call centers.

There are five million X1 set-top boxes installed in homes across the United States, though the company said the technical problems - now several days old - have not affected all of them.

The glitch was part of a software update, and Comcast said it was sending new software to the affected boxes that would "auto-heal" their TV service.

Comcast spokeswoman Jennifer Khoury said the company would evaluate those who "were impacted by the software update" and their accounts would be credited. She said the company did not yet have the crediting details.

"We know some of our customers may have missed their favorite shows off and on over the past few days and were unable to easily reach our customer care representatives for assistance . . . and we're really sorry," Charlie Herrin, the new senior vice president of customer experience, said in a blog post Thursday.

"While service has returned to normal for most X1 customers, our engineers are now going back over this issue and taking extra steps to prevent it from happening again," Herrin said.

Comcast said the fix should be automatic and does not require Xfinity subscribers to reboot or unplug the X1 set-top box.

On Thursday, the website Downdetector.com indicated outages in the Philadelphia, New York, and Chicago areas, and in California, Washington state, and Colorado.

By early evening, more than 7,000 comments had been filed on the website related to the Comcast outage.

"X-1 still down," one comment said. "Nothing seems to fix it. I am having it replaced with an older box in a week."

A woman from Port Charlotte, Fla., said, "Hope the company is planning on compensation for 1/3 of the US customers being without any TV for a few days!"

Comcast executives say X1 is the cable industry's leading technology and is central to its plan to modernize its telecommunications network.

Teri Simon, an Xfinity subscriber from Lower Merion, said she did not know whether she had an X1 set-top box but she had been without Internet since Tuesday night.

"Comcast needs better crisis management," Simon said. "You can't get anybody to say anything."