The United States and Western allies say they are investigating but can not confirm reports a blast in NATO member Poland resulted from stray Russian missiles - a claim Russia's defence ministry has denied.
Two people were killed in an explosion in Przewodow, a village in eastern Poland about 12 kilometres from the border with Ukraine, firefighters said.
The Associated Press earlier cited a senior US intelligence official as saying the blast was due to Russian missiles crossing into Poland.
Polish Radio ZET attributed the explosion to two stray missiles, without giving more details.
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.
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 Norway said it was seeking details.
Ukraine's President Volodymyr Zelenskiy said Russian missiles hit Poland in a "significant escalation" of the conflict.
He did not provide evidence of the strikes.
Russia's defence ministry denied 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."
The Kremlin did not immediately respond to requests for comments.
Russia was pounding cities across Ukraine with missiles on Tuesday in attacks Kyiv said were the heaviest wave of missile strikes in almost 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 it was too early to say whether the possible strike was intentional or accidental.
But the events were enough to trigger NATO treaty articles under which Poland would call a NATO meeting "to consult each other, to assess the threat and to take concrete action", Pothier said.
Polish Prime Minister Mateusz Morawiecki has called an urgent meeting of a government committee for national security and defence affairs, government spokesman Piotr Muller said on Twitter.
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."
© AP 2022