WARSAW, Poland (AP) — Polish and allied aircraft were deployed in a “preventive” operation in Poland's airspace Saturday because of a threat of drone strikes in neighboring areas of Ukraine, and the airport in the eastern Polish city of Lublin was closed, authorities said.

The alert came after multiple Russian drones crossed into Poland on Wednesday, prompting NATO to send fighter jets to shoot them down and underlining long-held concerns aboutthe expansion of Russia’s more than three-year war in Ukraine.

The Polish military’s operational command posted on X on Saturday afternoon that ground-based air defense and reconnaissance systems were on high alert. It stressed that “these actions are preventive in nature,” and were aimed at securing Poland's airspace and protecting the country's citizens. It cited a threat of drone strikes in regions of Ukraine bordering Poland, but didn't immediately give further details.

Prime Minister Donald Tusk also posted that “preventive air operations” had begun in Polish airspace because of the threat posed by Russian drones operating over nearby areas of Ukraine.

The Polish Air Navigation Services Agency said that Lublin Airport was closed to air traffic “due to military aviation activities.” It didn't specify how long that would last, but airport spokesman Piotr Jankowski told the PAP news agency that the airspace above the airport was closed until 6 p.m. (1600 GMT).

Russia has said it didn't target Poland on Wednesday, and Moscow’s ally, Belarus, said that the drones went astray because they were jammed. But European leaders have expressed certainty that the incursions were a deliberate provocation by Russia.