Russian missiles have killed two people in NATO member Poland, a senior US intelligence official told the AP news agency.
Polish Prime Minister Mateusz Morawiecki called an urgent meeting to discuss national security.
Afterwards, Polish government spokesman Piotr Muller said the country was increasing the readiness of its military units.
He also confirmed that there had been an explosion that killed two Polish citizens and said that discussions were under way over whether to trigger NATO’s Article Four, which involves all members meeting at the headquarters of the alliance
Ukrainian President Volodymyr Zelensky said Russian missiles had hit Poland and he had long warned that Russian actions were not limited to Ukraine.
He added that a strike on NATO territory was a “major escalation” and “action” was needed.
A NATO official said the alliance was investigating and coordinating closely with Poland.
European Council President Charles Michel said he was proposing that EU leaders attending the G20 summit in Bali hold a coordination meeting on Wednesday.
“We stand with Poland,” Michel tweeted.
US Pentagon spokesman Brigadier General Patrick Ryder said the US would “defend every inch of NATO territory”, but added that he had “no information to corroborate the press reports”. of the alleged Russian attack.
‘Crisis situation’ as NATO responds: follow the reaction live
Polish media said the missiles hit an area where grain was drying in Przewodow, a town in eastern Poland near the border with Ukraine.
“Firefighters are on the scene; it’s not clear what happened,” said firefighter Lukasz Kucy.
Russia’s defense ministry said reports of Russian involvement were a “deliberate provocation aimed at escalating the situation”.
He added in a statement: “There was no attack on targets near the Ukrainian-Polish state border with Russian means of destruction.”
However, Moscow launched a large number of missiles against Ukraine on Tuesday, which cut power to seven million homes.
Some of the missiles hit Lviv in western Ukraine, which is just 50 miles from the border with Poland.
Lord Richard Dannatt, former head of the British army, told Sky News: “Modern technology is quite precise, so it’s quite difficult to explain that this could have been an accident.
“If it wasn’t an accident, then it was a test of the West’s response, and that’s something that needs to be thought about very carefully.”
General Sir Richard Barrons, former head of the UK Joint Forces Command, said “the stakes are very high”.
He added that if Russia had deliberately targeted Poland, “this world of ours is taking a turn that no one could have imagined even a month ago.”
But General Sir Richard said it was “much more likely” that a “missile went rogue” and either malfunctioned or went astray.
Latvian Deputy Prime Minister Artis Pabriks said: “The Russian criminal regime fired missiles that not only targeted Ukrainian civilians but also landed on NATO territory in Poland.
“Latvia fully stands by Polish friends and condemns this crime.”
German Foreign Minister Annalena Baerbock said she was monitoring the situation closely and in contact with Polish friends and NATO allies.
Slovak Defense Minister Jaroslav Nad said he was “very concerned about the Russian missile drop in Poland”, adding: “Russia must explain what has happened. The senseless attacks on infrastructure have to stop immediately.
“Russia’s recklessness is spiraling out of control.”
Belgian Prime Minister Alexander De Croo tweeted: “We are all part of the NATO family.”
The Estonian foreign ministry called the news from Poland “most alarming” and said it was ready to defend “every centimeter of NATO territory”.
A UK Foreign Office spokesman said: “We are investigating these reports and are liaising closely with allies.”
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0:50 The US will “defend every inch of NATO territory”
NATO has a principle of collective defense which means that an attack against one ally is considered an attack against all allies.
Fabrice Pothier, NATO’s former director of planning, told Sky News that a NATO member that had been attacked could “trigger Article Five” and call on all other members to help it in its defense
He added that it was too early to say whether what happened in Poland was a “deliberate attack” or a “misfired missile”.
However, he said there were sufficient reasons to trigger Article Four.
Further actions could include increasing the air defenses of both Poland and Ukraine, Pothier added. He described Ukraine as “de facto the alliance’s first line of defense.”
Poland has not taken part in the conflict, but has taken in millions of Ukrainian refugees and widely condemned the war.