Polish authorities neutralized a drone flying over government buildings in Warsaw, Prime Minister Donald Tusk wrote in a Facebook post Monday.

Tusk said two Belarusian citizens had been detained. “Police are investigating the circumstances of the incident,” he added.

The episode comes after Poland shot down Russian drones that entered the country’s airspace on Sept. 10, putting NATO allies on high alert across the continent.

At least 19 drones entered Polish airspace that night, leading Poland to invoke Article 4 of the NATO treaty, which states that NATO allies will consult with one another when the territorial integrity of the alliance is threatened.

On Sept. 12, NATO Secretary-General Mark Rutte announced the launch of the “Eastern Sentry” mission to bolster defenses on the alliance’s eastern flank in response to the Russian drone incursion.

“While this was the largest concentration of violations of NATO airspace that we have seen, what happened on Wednesday was not an isolated incident,” Rutte said. “Russia’s recklessness in the air along our eastern flank is increasing in frequency. We have seen drones violate our airspace in Romania, Estonia, Latvia and Lithuania. Whether intentional or not, it is dangerous.”

Denmark, France, the U.K. and Germany have offered fighter jets, ships and air defense systems to the mission.