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.

Article 4 allows member states to start a formal discussion within the alliance about threats to their security. It does not commit the alliance to military action.

The Polish prime minister said there had been a serious violation of his country's airspace by Russian drones.

Multiple Russian drones crossed into Poland in what European officials described as a deliberate provocation, causing NATO to send fighter jets to shoot them down.

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.

Article 4 of the North Atlantic Treaty Organization is not as well known as its Article 5 collective security guarantee — which says an attack on all is an attack on one — but it is being increasingly invoked,

Lithuania is "ready" to shoot down any more drones that enter its airspace, the country's defense minister told ABC News.

Polish Prime Minister Donald Tusk told parliament it was "the closest we have been to open conflict since World War Two."

A continent already on edge over the Ukraine war sees a Russian challenge to NATO readiness and to an America that wants to disengage from Europe.

Russian drones and missiles have frequently breached NATO airspace in the past. Indeed, Ukrainian officials have complained that their allies have chosen to turn a blind eye to previous incursions. In late August a Shahed drone crashed in a cornfield in eastern Poland,