(Bloomberg) -- Increased Russian efforts to jam Global Positioning System signals in the aftermath of a number of drone attacks have forced airline pilots flying in the Baltic region and Finland to resort to alternative navigation methods. 

GPS signals across most of Estonia have been hit by interference in recent days, the country’s transportation agency said in an email.

Air traffic has been affected but ground signals are still working despite the jamming, which appears to come from Russia’s Leningrad region, Estonia’s consumer protection agency said in a separate email. That region surrounds Russia’s second-largest city, St. Petersburg, and shares a border with Estonia. 

The increased jamming comes after Russia was hit by drones several times in recent weeks, including a strike that caused a fire at an oil refinery in the country’s south and the biggest attack on Moscow since President Vladimir Putin invaded Ukraine. The Kremlin has blamed Kyiv for the incidents, while Ukraine has denied responsibility for certain attacks and remained silent on others. 

The Russian Defense Ministry did not immediately respond to a request for comment. 

Read More: Putin Orders Tighter Defenses After Drone Strikes on Moscow

“Jamming GPS signals is one of the possible measures of the Russian Federation to ensure the protection of important facilities within their country,” Estonia’s Defense Ministry said in an email. “In such cases, situations may arise where the jamming of signals also affects the use of GPS in nearby areas.”

Finland’s air traffic control said in an email that aircraft in the country are using alternate systems and that commercial flights have not been interrupted.

--With assistance from Aaron Eglitis.

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