Poland has sent military equipment to the border with Belarus after a Russian drone strike overnight prompted NATO fighter jets, activated air defenses, and closed Warsaw airports.

Footage of Polish military trucks moving along public roads was shown on state-run First National television on Wednesday morning, Interfax reports.

Polish Prime Minister Donald Tusk said Warsaw would invoke Article 4 of the NATO Charter, which allows for emergency consultations in the event of a threat to the security, territorial integrity, or political independence of any member state.

According to Tusk, at least 19 drones entered Polish airspace overnight. The Polish interior ministry said seven downed drones and a missile fragment had been found in the country. Five of the drones fell in the Lublin Voivodeship. One of them fell on a residential building in Wyryki village, near the Belarusian border.

The overnight incident brought Poland “closer to a military conflict than at any time since World War II,” Tusk told the Polish parliament. “I have no reason to say that we are on the brink of war, but the border has been crossed and it is much more dangerous than before.”

The Belarusian defense ministry said these drones were “stray.” Some of them were “destroyed by the republic’s air defense forces,” said First Deputy Defense Minister Pavel Muraveiko.

Since the start of the war in Ukraine, there have been several incursions by Russian drones into Polish and Romanian airspace, but so far Poland had not been forced to shoot them down. Ukrainian President Volodymyr Zelenskyy said that Russia had used 415 drones and 40 missiles to attack Ukraine overnight, and added that some of them were “aimed at Poland.”