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.

The United States told the United Nations Security Council on Friday it would "defend every inch of NATO territory" after a suspected Russian drone incursion into Poland.

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.

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.

Poland has triggered Article 4 of the North Atlantic Treaty Organization, NATO, after the country reported 19 “breaches” of its airspace by Russian drones on Wednesday.

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

President Donald Trump reacted Wednesday with a degree of bemusement toward Russia’s drone incursion into Poland, a NATO member that Trump has previously vowed to defend amid heightened tensions with Moscow.

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