Russian missiles have killed two people in NATO member Poland, a senior US intelligence official has told the AP news agency.
Polish Prime Minister Mateusz Morawiecki has called an urgent meeting to discuss national security, a government spokesman said on Twitter.
State news agency PAP said the meeting was to be held at 8pm GMT.
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.”
The Estonian foreign ministry said the news from Poland was “most alarming” and said 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 as an attack against all allies.
Fabrice Pothier, former director of planning at NATO, told Sky News that a NATO member that 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, which involves NATO members meeting at the alliance’s HQ to discuss the perceived threat “and to take concrete actions”.
Those 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”.
A US Pentagon spokesman said he had “no information to corroborate press reports” but was “taking them seriously and looking into them”.
A UK Foreign Office spokesman said: “We are investigating these reports and liaising closely with allies.”
Polish media said missiles hit an area where grain was drying in Przewodow, a village in eastern Poland near the border with Ukraine.
“Firefighters are on the spot – it’s not clear what has happened,” said firefighter Lukasz Kucy.
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.
It is unclear whether the two missiles that apparently landed in Poland were part of the same wave, but it is the first time Russian rockets are said to have reached Poland.
Poland has not been involved in the conflict, but has welcomed millions of Ukrainian refugees and has widely condemned the war.
