Websites of some major US airports went down early Monday morning in what appears to be a coordinated denial-of-service attack, although officials said flights were not affected.

The attacks came following a call from a mysterious group of pro-Russian hackers calling themselves Killnet to launch coordinated denial-of-service attacks on targets. The group posted the list of targets on its Telegram channel.

Andrew Jobel, spokesperson for Hartsfield-Jackson Atlanta International Airport, said, "We noticed this morning that the external website is down, and our IT and security staff are investigating."

"There was no impact on operations." Spokeswoman Victoria Spilabot said parts of the public-facing side of Los Angeles International Airport's website were also disrupted.

"Internal airport systems were not hacked and there were no operational disruptions." Spilabot said the airport notified the FBI and the Transportation Security Administration, and the airport's IT team was working to restore all services and investigate the cause.

Several other airports reported connectivity issues with their websites and appeared to be running very slowly, including Chicago O'Hare International Airport's site, which was included in Killnet's target list.

Chicago's aviation department said in a statement that the websites for O'Hare and Midway airports went down early Monday but no airport operations were affected. Last week, a group of hackers claimed responsibility for cyberattacks against state government websites across the country.

Editing: Yusra Bamtarf