"A full assessment is ongoing. But of course, whether it was intentional or not, it is absolutely reckless, it is absolutely dangerous," alliance chief Mark Rutte said after NATO members held emergency discussions.

Rutte said that NATO's "air defences were activated and successfully assured the defence of NATO territory, as they are designed to do".

"To Putin, I mean, my message is clear: stop the war in Ukraine, stop the escalating war, which he is now basically mounting on innocent civilians and civilian infrastructure," he added.

"Stop violating allied airspace, and know that we stand ready, that we are vigilant, and that we will defend every inch of NATO territory," he said.