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All of United Airlines' domestic flights were grounded for more than two hours Sunday night because of a computer outage, the Federal Aviation Administration said as scores of angry travelers sounded off on social media.

International flights weren't affected by the ground stop, which the FAA said was issued at United's request.

Travelers gather during the delay of a United Airlines flight Sunday in San Diego. Austin Bales

"We're working to get flights on their way," United said in a statement shortly after 9 p.m. ET. "We apologize for the inconvenience to our customers."

U.S. officials told NBC News that the Aircraft Communications Addressing and Reporting System, or ACARS, had issues with low bandwidth. No further explanation was immediately available for what United described only as "an IT issue."

United airlines is experiencing technical problems this evening. Please check your flight status to see if your flight is affected. — Denver Int'l Airport (@DENAirport) January 23, 2017

I so hate leaving my kids & then @united makes it worse by giving me my 8th cancellation or… https://t.co/WWmY9ESLHT — Christopher Cornue (@cornue) January 23, 2017

Massive computer outage for @united causing delays all over the map. Currently stuck on tarmac in Denver. Damn you, Putin! — Eric Timm (@Tymer_T) January 23, 2017

Just my luck. Flying the day there's a system outage on @united ???? — Mekanikal (@DJMekanikal) January 23, 2017

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