After multiple Russian drones violated NATO airspace, Europe faces questions over the alliance’s long-term ability to deal with this growing threat.

Polish President Karol Nawrocki on Sunday signed a classified decree permitting a permanent NATO troop in Poland as part of Eastern Sentry.

Russian Telegram channels have reported that explosions were heard in the city of Saratov on the night of 19-20. A fire has likely occurred at an oil refinery. Source: Astra and other Telegram channels Details: Explosions were heard in Russia's Saratov.

In the week since NATO fighter jets scrambled to shoot down multiple dozens of Russian drones that had crossed into alliance airspace in Poland, US and Western intelligence officials have been unable to determine whether the incursion was accidental or an intentional effort by Russia to probe western air defenses and gauge NATO’s response.

Drones used for defense and attack have taken a central battlefield role in the more than three years since Russia invaded Ukraine.

Ukraine has ramped up strikes on energy sites to increase pressure on Russia, which relies on revenue from oil and gas exports.

Russia continued its nightly strikes on Ukraine -- including the capital Kyiv -- as U.S. President Donald Trump prepared to depart for his state visit to the U.K.

Ukraine is struggling to defend itself against the growing number of attack drones that Moscow has started using in its onslaughts.

"If it had all happened at once, maybe you could call it an accident. But six hours of drones? That’s a test – a way to see what the West would do. And we have to respond like it’s a test," Kellogg said.