The websites of a number of major U.S. airports were briefly knocked offline Monday after a cyberattack promoted by a pro-Russian hacking group.

The disruptions affected airport websites in cities such as Atlanta, Chicago, Los Angeles, New York, Phoenix and St Louis, and were carried out via distributed denial of service (DDoS) attacks, which involve taking a website offline by flooding it with traffic.

The airport websites were targeted after the pro-Russian hacking group known as "KillNet" published a list of sites and encouraged its followers to attack them.

The DDoS attacks only affected the public-facing websites of the airports, which supply flight and service information and do not have any impact on operations.

Atlanta's Hartsfield-Jackson Airport said its website is "up and running after an incident early this morning that made it inaccessible to the public."

"An investigation into the cause of the incident is underway," it said. "At no time were operations at the airport impacted."

Most of the airport websites targeted appeared to be functioning normally after being temporarily shut down.

KillNet claimed responsibility last week for attacks on a number of U.S. state government websites, and it has taken aim at other countries opposing Russia's invasion of Ukraine.

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) said it was "aware of reports of DDoS attacks targeting multiple U.S. airport websites."

"We are coordinating with potentially impacted entities and offering assistance as needed," a CISA spokesperson said.