Poland shot down suspected Russian drones in its airspace on Wednesday with the backing of military aircraft from its Nato allies, the first time a member of the Western military alliance is known to have fired shots during Russia's war in Ukraine.
Polish Prime Minister Donald Tusk told parliament it was "the closest we have been to open conflict since World War Two", although he also said he had "no reason to believe we're on the brink of war".
Polish F-16 fighter jets, Dutch F-35s, Italian AWACS surveillance planes and Nato mid-air refuelling aircraft scrambled in an operation to shoot down drones that began entering Polish airspace on Tuesday evening and kept coming until morning, officials said.
One drone smashed into Tomasz Wesolowski's two-storey brick house in the eastern Polish village of Wyryki-Wola at 6:30am while he was downstairs watching news about the incursion on TV.
The roof was destroyed and debris was strewn across the bedroom. Wesolowski told Reuters the house "needs to be demolished".
A blackened spot in a black field elsewhere in southeastern Poland showed where other drones had fallen.
Moscow denied responsibility for the incident, with a senior diplomat in Poland saying the drones had come from the direction of Ukraine. Russia's defence ministry said its drones had carried out a major attack on military facilities in western Ukraine but it had not planned to hit any targets in Poland.
The leaders of France, Britain, Germany and Canada were among Nato leaders to condemn the Russian incursion in strong terms.
There was no immediate statement from Washington. The US Air Force general serving as the top Nato commander, Alexus Grynkewich, said the alliance had "responded quickly and decisively to the situation, demonstrating our capability and resolve to defend allied territory".
European leaders, who have lately been trying to persuade US President Donald Trump to join them in tightening sanctions on Russia and boosting support for Kyiv, said it justified a collective response.
Poland said 19 objects had entered its airspace during a large Russian air attack on Ukraine, and that it had shot down those that posed a threat.
Tusk called the incident a "large-scale provocation" and said he had activated Article Four of Nato's treaty, under which alliance members can demand consultations with their allies.
Andrey Ordash, Russia's charge d'affaires in Poland, was cited by RIA state news agency as calling accusations of an incursion "groundless" and said Poland had not given any evidence that the drones shot down were of Russian origin.
The Kremlin declined to comment directly on the incident but spokesperson Dmitry Peskov said the EU and Nato "accuse Russia of provocations on a daily basis. Most of the time without even trying to present at least some kind of argument."
During the incident, the Operational Command of the Polish Armed Forces urged residents to stay at home, with three eastern regions at particular risk.
Several Polish airports were temporarily closed, including one that has been used as the main access point for Western officials and supplies travelling to Ukraine over land.
Nato chief Mark Rutte said that a full assessment of the incident had not yet been made but added that, "whether it was intentionally or not, it is absolutely reckless, it is absolutely dangerous."
Countries bordering Ukraine have reported occasional Russian missiles or drones entering their airspace in the past during the war, but not on such a large scale, and they are not known to have shot them down. Two people were killed in Poland in 2022 by a Ukrainian air defence missile that went astray.
Since Nato's creation in 1949, Article 4 has been invoked seven times, most recently in February 2022 following Russia's invasion of Ukraine.
"Russia's war is escalating, not ending. We must raise the cost on Moscow, strengthen support for Ukraine, and invest in Europe's defence," the EU's top diplomat Kaja Kallas said.
Early indications suggested the entry of Russian drones into European airspace was intentional, not accidental, she added in her post on X.
Ukrainian President Volodymyr Zelenskiy said Russia used 415 drones and 40 missiles in attacks on Ukraine overnight, adding that at least eight Iranian-made shahed drones had been aimed towards Poland.
"An extremely dangerous precedent for Europe," he said. "A strong response is needed – and it can only be a joint response by all partners: Ukraine, Poland, all Europeans, the United States."
Russia has long said it has no intention of stoking a war with Nato and that Western European countries suggesting it is a threat were trying to worsen relations.
European Commission President Ursula von der Leyen called for more sanctions on Russia, and said the EU was preparing sanctions on 'shadow fleet' tankers that transport its oil and third countries that buy it.
Trump, who warmly welcomed Putin in Alaska at a summit in August, said over the weekend he was ready to move to a second phase of sanctioning Russia after months of talks about a peace deal.