Two people were killed in an explosion in Przewodow, a village in eastern Poland near the border with Ukraine, firefighters said on Tuesday as NATO allies investigated reports that the blast resulted from Russian missiles.
The Associated Press earlier cited a senior U.S. intelligence official as saying the blast was due to Russian missiles crossing into Poland.
But the Pentagon said it could not confirm that account.
The White House also said it could not confirm reports coming out of Poland and was working with the Polish government to gather more information.
RepublikaTV quickly tweeted video of the suspected strike.
Russia's defense ministry denied reports that Russian missiles had hit Polish territory, describing them 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.
Polish Prime Minister Mateusz Morawiecki has called an urgent meeting of a government committee for national security and defense affairs, government spokesman Piotr Muller said on Twitter.
Latvian Deputy Prime Minister Artis Pabriks said on Twitter that Russia "fired missiles which target not only Ukrainian civilians but also landed on NATO territory in Poland."
Officials from Norway, Lithuania and Estonia - members of the NATO defense alliance - said they were trying to find out more information. "This is a very serious incident but much remains unclear," Norwegian Foreign Minister Anniken Huitfeldt said, according to Norwegian news agency NTB.
Members of the North Atlantic Treaty Organisation (NATO) are committed to collective defense, and the possibility that the Polish explosion resulted from an intentional or accidental Russian strike raised alarm.
"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."
Polish Radio ZET reported earlier that two stray missiles hit Przewodow, killing two people, without giving any more details.
"Firefighters are on the spot, it's not clear what has happened," said Lukasz Kucy, an officer on duty at a firefighters' post near the Polish village.
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.
State-run news agency PAP reported that the Polish government would hold a meeting at 2000 GMT.
Muller told reporters later on Tuesday that the committee was convened due to the "crisis situation," adding that relevant information will be presented to the public later. He called on media not to publish "unverified information" in the meantime. (Reporting by Marek Strzelecki, Anna Koper, Pawel Florkiewicz; Writing by Alan Charlish and Cynthia Osterman; Editing by Chizu Nomiyama, Emelia Sithole-Matarise and Sandra Maler)
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