Russia is being accused of a missile strike that killed two people in Poland.
Here's what we know about the developing situation that has caused alarm across the world.
What has happened?
Polish government spokesman, Piotr Muller, said there had been an explosion that killed two Polish citizens, and that discussions were under way over whether to activate NATO's Article 4, which involves all members meeting at the alliance's HQ.
Polish media reported that missiles hit an area where grain was drying in the eastern village of Przewodow, near the Ukraine border.
Poland's defence ministry said a Russian-produced rocket fell on the village at 3.40pm local time (1.40pm UK time).
Local firefighter Lukasz Kucy told reporters: "Firefighters are on the spot - it's not clear what has happened."
The specifics of the explosion have not yet been established and Polish Prime Minister Mateusz Morawiecki called an urgent meeting to discuss national security.
Poland has also raised the readiness of its military units and the country's president, Andrzej Duda, has spoken with US President Joe Biden and Ukraine's President Volodymyr Zelenskyy.
What has Russia said?
Russia's defence ministry has described reports of Moscow's involvement 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."
However, a large number of Russian missiles had been launched at Ukraine on Tuesday, with several hitting the city of Lviv, just 50 miles from the Polish border.
What has NATO said?
NATO Secretary-General Jens Stoltenberg spoke to Polish President Andrzej Duda about the explosion in the east of the country late on Tuesday.
"I offered my condolences for the loss of life," he said in a statement.
"NATO is monitoring the situation and allies are closely consulting. Important that all facts are established."
The bloc has a principle of collective defence, which means that an attack against one is considered an attack against all, raising the prospect of a stark escalation to the conflict.
Poland has not been directly involved in the war, but has welcomed millions of Ukrainian refugees and widely condemned Russia's aggression.
What has Ukraine said?
President Zelenskyy said he had long warned that Russia's actions were not limited to Ukraine.
He added that any strike on a NATO country was a "significant escalation" and called for "action".
What happens next?
A NATO official said the alliance was investigating and coordinating closely with Poland.
If Poland invokes Article 4 of the treaty, NATO ambassadors will meet at the North Atlantic Council.
According to Article 4 of the alliance's founding treaty, members can raise any issue of concern, especially related to the security of a member country.
A more extreme measure would be to invoke Article 5, which provides that if a NATO Ally is the victim of an armed attack, each and every other member of the alliance will consider this act of violence as an armed attack against all members and will take the actions it deems necessary to assist.
How has the world reacted?
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.
UK Foreign Secretary James Cleverly tweeted: "We are urgently looking into reports of missiles landing in Poland, and are in contact with our Polish friends and NATO allies."
The president of the European Council, Charles Michel, said he was proposing that EU leaders attending the G20 summit in Bali should hold a coordination meeting on Wednesday.
Germany's foreign minister, Annalena Baerbock, said she was monitoring the situation closely and in contact with Poland 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".