Nato member Poland said on Wednesday that a Russian-made rocket killed two people in eastern Poland near Ukraine, and it summoned Russia's ambassador to Warsaw for an explanation after Moscow denied it was responsible.
The Polish foreign ministry said the rocket fell on Tuesday afternoon on Przewodow, a village in eastern Poland about 6km from the border with Ukraine, killing two. Media reports said the strike hit a grain-drying facility.
The statement was Poland's most detailed comment thus far on the explosion, which struck at 3.40pm. The US and Western allies had said they were investigating but could not confirm a report that stray Russian missiles hit the village.
Members of the North Atlantic Treaty Organization (Nato) are committed to collective defence, so a Russian strike on Poland could risk widening the conflict between Russia and Ukraine, which began with Moscow's invasion in February.
A Nato official said the alliance was looking into the report and closely coordinating with Poland.
Poland was increasing the readiness of some military units and determining whether to request consultations with allies under Article 4 of the Nato treaty, Polish Prime Minister Mateusz Morawieck said.
US President Joe Biden told Polish President Andrzej Duda in a call that Washington has an "ironclad commitment to Nato" and will support Poland's investigation, the White House said.
The Associated Press earlier cited a senior US intelligence official as saying the blast was due to Russian missiles having crossed into Poland.
But in Washington, the Pentagon, White House and US State Department said they could not corroborate the report and were working with the Polish government to gather more information. The State Department said the report was "incredibly concerning."
Germany and Canada said they were monitoring the situation, and the European Union, the Netherlands and Norway said they were seeking more details. French President Emmanuel Macron ordered a verification effort, while Britain was "urgently" looking into the report.
Ukraine's President Volodymyr Zelenskiy said Russian missiles hit Poland in a "significant escalation" of the conflict. He did not provide evidence.
Russia's defence ministry denied that Russian missiles hit Polish territory, describing reports as "a deliberate provocation aimed at escalating the situation".
It added in a statement: "No strikes on targets near the Ukrainian-Polish state border were made by Russian means of destruction."
Kremlin spokesman Dmitry Peskov said he had no information on an explosion in Poland.
Russia was pounding cities across Ukraine with missiles on Tuesday, in attacks that Kyiv said were the heaviest wave of missile strikes in nearly nine months of war. Some hit Lviv, which is less than 80km from the border with Poland.
Fabrice Pothier, former head of policy planning in the Nato secretary-general's office, told Sky TV that the events were enough to trigger Nato's Article 4. That would entail Poland calling a Nato meeting "to consult each other, to assess the threat and to take concrete action," Pothier said. Nato ambassadors were due to hold a regular weekly meeting on Wednesday.
Latvian Deputy Prime Minister Artis Pabriks said the situation was "unacceptable" and it could lead to Nato providing more anti-aircraft defences to Poland and Ukraine, a view Pothier endorsed.
"Every inch of #Nato territory must be defended!," Lithuanian President Gitanas Nauseda said on Twitter.
Estonian Foreign Minister Urmas Reinsalu said, according to BNS newswire: "We are discussing with our allies how to respond to what happened jointly and decisively."