A Russian flight booking system was hit by a cyberattack on Thursday, causing delays at airports.

A "massive" distributed denial-of-service (DDoS) attack on the local airline booking system Leonardo was carried out by "foreign hackers," reported one of the system’s developers, Russian state defense company Rostec.

The incident lasted about an hour and affected the operation of several Leonardo customers, including Russian air carriers Rossiya Airlines, Pobeda and flagship airline Aeroflot. DDoS attacks overwhelm websites with a flood of traffic, making them temporarily unavailable to users.

According to Aeroflot, the incident caused delays of up to an hour for departures at Moscow's Sheremetyevo International Airport — the busiest in the country.

Leonardo is used by more than 50 Russian carriers and serves around 45 million passengers annually, according to the Russian news agency Interfax.

The Ukrainian hacktivist group IT Army claimed responsibility for shutting down Leonardo.

“While you [Ukrainians] are sipping lattes, our friends up north are stuck in queues, trying to book flights,” the hackers reported on their Telegram channel. “Well done IT Army!”

According to Rostec, Leonardo is constantly experiencing "large-scale and unprecedented" attacks.

The flight booking system has “encountered dozens of these incidents in recent months, with around five attempts recorded in September," the agency stated.

"It's clear that there's an actual cyberwar waged against Russia. Its goal is to damage the Russian IT infrastructure and disrupt the work of the most important industries.”

Earlier this month, a cyberattack suspected to be carried out by a pro-Russia hacking group caused widespread service disruptions at several Canadian airports. On the same day, Canada’s largest airline announced a data breach that involved the information of employees.

No group has claimed responsibility for the attack on Air Canada, but the country’s businesses have suffered relentless attacks from Russian threat actors since Canada’s government announced its support for Ukraine last year.