(NBC News) — At least 13 U.S. air travel websites, including those for Hartsfield-Jackson Atlanta International Airport and Los Angeles International Airport — two of the busiest airports in the country — were inaccessible Monday morning after a Russian hacktivist group named them as targets for cyberattacks.

There is no indication that any airport operations were affected, and the type of cyberattack the hackers claimed to use doesn’t do any lasting damage. But it was a sign of how an increasingly effective pro-Russia group, Killnet, can cause mischief for U.S. websites. Last week, Killnet targeted the websites of several U.S. states, successfully knocking Colorado.gov offline for more than a day and briefly interrupting Kentucky.gov.

Killnet specializes in Distributed Denial of Service attacks, or DDoS attacks, which overwhelm a website with internet traffic. While DDoS attacks are generally considered little more than a nuisance, they can knock websites offline for hours or even days.

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