The United States and Western allies said they were investigating but could not confirm a report on Tuesday that a blast in NATO member Poland resulted from stray Russian missiles, while Russia's defence ministry denied it.
Two people were killed in an explosion in Przewodow, a village in eastern Poland about 6 kilometers (3-1/2 miles) from the border with Ukraine, firefighters said. Media reports said the strike hit a grain-drying facility.
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 was 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, spokesman Piotr Muller said. Polish President Andrzej Duda and U.S. President Joe Biden were speaking, officials said.
The Associated Press earlier cited a senior U.S. intelligence official as saying the blast was due to Russian missiles having crossed into Poland.
Polish Radio ZET attributed the explosion to two stray missiles, without giving more details.
But in Washington, the Pentagon, White House and U.S. 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.
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 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."
The Kremlin did not immediately respond to requests for comments.
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 80 km (50 miles) 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's Article 4 under which Poland would call 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.
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."
(Reporting by Marek Strzelecki, Anna Koper, Pawel Florkiewicz; Additional reporting by Sabine Siebold in Berlin; Writing by Alan Charlish and Cynthia Osterman; Editing by Chizu Nomiyama, Emelia Sithole-Matarise and Sandra Maler)
