Poland shot down drones that entered its airspace Wednesday, marking the first known instance of a NATO member firing weapons during Russia's war in Ukraine.
Warsaw 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.
Prime Minister Donald Tusk said he had activated Article 4 of NATO's treaty, under which alliance members can demand consultations with their allies.
"We are dealing with a large-scale provocation," said Tusk. "We are ready to repel such provocations. The situation is serious, and no one doubts that we must prepare for various scenarios."
Russia's RIA state news agency quoted a Russian diplomat as calling the accusations of an incursion "groundless" and saying Poland had not given any evidence that the drones shot down were of Russian origin.
Several European officials described the incursion as intentional and a sign of Russian escalation.
"The fact that these drones, which posed a security threat, were shot down changes the political situation," said Tusk.
The Operational Command of the Polish Armed Forces urged residents to stay at home, with three eastern regions at particular risk.
"This is an act of aggression that posed a real threat to the safety of our citizens," it said.
The incident could feed into deliberations on tightening sanctions on Moscow. European countries are hoping to persuade U.S. President Donald Trump to join them in imposing sanctions after sporadic peace talks on ending the war proved fruitless.
Countries bordering on 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 defense missile that went astray.
A NATO spokesperson said NATO chief Mark Rutte was in touch with Polish leadership and the alliance was consulting closely with Poland. A source said NATO was not treating the incident as an attack but as an intentional incursion.
Polish F-16 fighter jets, Dutch F-35, Italian AWACS surveillance planes and mid-air refueling aircraft jointly operated by NATO were involved in the overnight operation, according to the source.
Poland's military command said radars had tracked more than 10 objects and those that could pose a threat were "neutralized." By morning, it said operations had concluded.
Early indications suggested the entry of Russian drones into European airspace was intentional, not accidental, the EU's top diplomat Kaja Kallas said Wednesday.
"Russia's war is escalating, not ending. We must raise the cost on Moscow, strengthen support for Ukraine, and invest in Europe's defense," Kallas said. "The EU plays a major role and we will support initiatives like the Eastern Border Shield defense line," Kallas wrote in a post on X.
U.S. Secretary of State Marco Rubio had been briefed, CNN reporter Kaitlan Collins said Tuesday. The State Department did not immediately respond to a request for comment.
Ukrainian President Volodymyr Zelenskyy 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 toward 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 are trying to worsen relations.
President of the European Commission 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.
Czech Foreign Minister Jan Lipavsky called for NATO to strengthen air defences on its frontline.
Chopin Airport in Warsaw, the country's largest, closed its airspace for several hours before reopening. It said there would be disruptions and delays through the day. The airport in the city of Lublin, in eastern Poland, remained closed.
Most of Ukraine, including western regions of Volyn and Lviv which border Poland, had been under air raid alerts nearly all night, according to Ukraine's air force.
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.
It was his strongest indication yet that he may escalate pressure on Moscow or its oil buyers in response to the war in Ukraine.
The European Union's top sanctions official has been in Washington this week to discuss coordinated sanctions. If such are adopted, it would be the first time the United States and Europe had taken coordinated steps against Russia since Trump returned to office in January, promising to end the war in 24 hours.
