NATO has launched an initiative to reinforce the defense of Europe’s eastern flank after a Russian drone incursion into Polish airspace this week, NATO Secretary-General Mark Rutte announced on Friday.

In the years since Russia invaded Ukraine, NATO has focused on defending its own territory first and resisted lower-level harassment from Russia, like acts of sabotage and intimidation.

NATO Secretary-General Mark Rutte called Russia's behavior "reckless" after several drones from the country crossed into Poland, violating NATO airspace.

The incident represented the most serious spillover of the war in Ukraine for NATO, as its aircraft engaged the drones that it said came from Russia.

Polish F-16 fighters, Dutch F-35 fighters, German Patriot air defense systems and an Italian AWACS surveillance aircraft were all deployed, and three of the drones were downed. Others crashed, including near villages in eastern Poland.

Poland has called for a meeting with NATO officials after invoking Article 4 of the alliance's charter and will review the incident and share the findings with its allies. Meanwhile, the fallout from the airspace breach continues to heighten tensions between Russia and the West.

Nato fighter jets shot down Russian drones over Poland in the first ever direct engagement between the Western alliance and Moscow.