A Russian-made missile killed two people in Poland Tuesday, Warsaw said, as it put its military on heightened readiness in a potentially major escalation of the war in Ukraine.
Warsaw said the missile killed two people in the village of Przewodow, but did not have conclusive evidence of who fired it, adding that Moscow's ambassador has been summoned to provide "immediate detailed explanations".
Warsaw put its military on heightened alert after an emergency national security council meeting.
"There has been a decision to raise the state of readiness of some combat units and other uniformed services," spokesman Piotr Muller told reporters after the meeting in Warsaw, adding that "our services are on the ground at the moment working out what happened."
President Joe Biden spoke by phone with his Polish counterpart Andrzej Duda, offering "full US support for and assistance with Poland's investigation", the White House said.
The two leaders agreed to "remain in close touch to determine appropriate next steps as the investigation proceeds", it added.
Poland is protected by NATO's commitment to collective defence -- enshrined in Article 5 of its founding treaty -- but the alliance's response will likely be heavily influenced by whether the incident was accidental or intentional.
Biden also spoke with NATO chief Jens Stoltenberg about the blast in Poland, while ambassadors from the alliance were to hold an emergency meeting on Wednesday.
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The incident drew widespread condemnation, with European Union chief Charles Michel saying he was "shocked" and French President Emmanuel Macron calling for talks at the G20 summit underway in Indonesia.
Ukrainian President Volodymyr Zelensky had earlier said two Russian missiles hit Poland in what he described as "a very significant escalation."
Ukraine's Foreign Minister Dmytro Kuleba rejected as a "conspiracy theory" the idea that the Poland blast may have been caused by a surface-to-air missiles fired by Kyiv's forces, while Russia's defence ministry dismissed reports that it was to blame as a "provocation" intended to escalate tensions.
The blast came after Russian missiles hit cities across Ukraine Tuesday, including Lviv near the border with Poland.
Zelensky said the strikes cut power to some 10 million people, though it was later restored to eight million of them, and also triggered automatic shutdowns at two nuclear power plants.
He said Russia had fired 85 missiles at energy facilities across the country, condemning the strikes as an "act of genocide" and a "cynical slap in the face" of the G20.
Moldova, which also borders Ukraine, reported power cuts because of the missiles fired at its neighbour and called on Moscow to "stop the destruction now".