A Russian missile has killed two people in NATO member Poland, a senior US intelligence official has told the AP news agency.
Polish Prime Minister Mateusz Morawiecki called an urgent meeting to discuss national security.
The Polish foreign ministry later said that a Russian-produced rocket fell on the village of Przewodow at 3.40pm local time, near the border with Ukraine.
The ministry added that it had summoned the Russian ambassador. Moscow has denied any involvement.
Mr Morawiecki said he was calling on all Poles to remain calm and air space monitoring was being stepped up.
Poland is raising the readiness of its military units, government spokesman Piotr Muller said.
He also confirmed there had been an explosion that killed two Polish citizens.
Polish President Andrzej Duda later said it was "very likely" that his country would trigger NATO's article four during a meeting of the alliance's 30 member states on Wednesday.
Under article four, members can raise a security issue and ask for it to be discussed.
Mr Duda added that there was no "definite evidence" of who had fired the rocket, describing it as a one-off incident, with no indications it would be repeated.
He said it was "most probably" Russian-made, but that is still being verified.
Ukrainian President Volodymyr Zelenskyy said a strike on NATO territory was a "significant escalation" and "action" was needed.
Mr Duda has spoken to US President Joe Biden, Mr Zelenskyy, and UK Prime Minister Rishi Sunak.
Mr Sunak said he had "reiterated the UK's solidarity with Poland".
British Foreign Secretary James Cleverly said he was in contact with Polish counterparts and NATO allies.
The president of the European Council, 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," Mr Michel tweeted.
US Pentagon spokesman Brigadier General Patrick Ryder said America would "defend every inch of NATO territory", but added that he had "no information to corroborate press reports" of the alleged Russian attack.
Polish media said a missile had hit an area where grain was drying in Przewodow.
"Firefighters are on the spot - it's not clear what has happened," said firefighter Lukasz Kucy.
Russia's defence ministry said reports of Russian involvement were 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."
However, Moscow launched a large number of missiles at Ukraine on Tuesday, knocking out power for seven million households.
Some of the missiles hit Lviv in western Ukraine, which is only about 50 miles from the Polish border.
Lord Richard Dannatt, former head of the British Army, told Sky News: "Modern technology is pretty accurate, so it's quite difficult to explain away that this might 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's got to be thought through very carefully."
General Sir Richard Barrons, former head of the UK's Joint Forces Command, said the "stakes are really high".
He added that if Russia had targeted Poland deliberately then "this world of ours is taking a turn that no one could possibly have imagined even a month ago".
But General Sir Richard said it was "far more likely" that a "missile had gone rogue" and had malfunctioned or been deflected.
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Latvia's deputy prime minister, Artis Pabriks, said: "Criminal Russian regime fired missiles which target not only Ukrainian civilians but also landed on NATO territory in Poland.
"Latvia fully stands with Polish friends and condemns this crime."
Germany's foreign minister, Annalena Baerbock, said she was monitoring the situation closely and had been in contact with Polish friends and NATO allies.
Slovak Defence Minister Jaroslav Nad said he was "very concerned by Russian missiles dropping in Poland", adding: "Russia must explain what happened. Senseless attacks on infrastructure must stop immediately.
"Russia's recklessness is getting out of hand."
Belgian Prime Minister Alexander De Croo tweeted: "We are all part of the NATO family."
The Estonian foreign ministry said the news from Poland was "most alarming" and it was ready to defend "every inch of NATO territory".
NATO has a principle of collective defence which means that an attack against one ally is considered an attack against all allies.
Fabrice Pothier, former director of planning at NATO, told Sky News that a NATO member which had been attacked could "trigger article five" and call all other members to help in its defence.
He added that it was too early to say whether what happened in Poland was an "intended attack" or whether it was the "misfiring of a missile".
However, he said there was enough reason to trigger article four.
Subsequent actions could include augmenting the air defences of both Poland and Ukraine, Mr Pothier added. He described Ukraine as "de facto the first line of defence of the alliance".
Poland has not been involved in the conflict, but has welcomed millions of Ukrainian refugees and has widely condemned the war.