The explosion in Poland came as Russia pounded cities across Ukraine with missiles, in attacks that Kyiv said were the heaviest wave of strikes in nearly nine months of war. Some hit Lviv, which is less than 80 km (50 miles) from the border with Poland.

Ukrainian President Volodymyr Zelenskiy said Russian missiles hit Poland in a “significant escalation” of the conflict. He did not provide evidence of Russia’s involvement.

“All of Europe and the world must be fully protected from terrorist Russia,” he said in a tweet after a phone call with Polish President Andrzej Duda.

Two European diplomats said Poland requested a NATO meeting under the treaty’s Article 4 for consultations among the allies. Poland was also increasing the readiness of some military units, Polish Prime Minister Mateusz Morawiecki said.

Polish officials sought to avoid inflaming the situation. Morawiecki called on all Poles to remain calm, and President Duda said there was no concrete evidence showing who fired the missile and that it was a one-off incident.

Biden told Duda in a call that Washington has an “ironclad commitment to NATO” and will support Poland’s investigation, the White House said.

The Associated Press earlier cited a senior US intelligence official as saying the blast was due to Russian missiles having crossed into Poland.

But in Washington, the Pentagon, White House and US State Department said they could not corroborate the report and were working with the Polish government to gather more information. The State Department said the report was “incredibly concerning.”

Germany and Canada said they were monitoring the situation, and the European Union, the Netherlands and Norway said they were seeking more details. French President Emmanuel Macron ordered a verification effort, while Britain was “urgently” looking into the report.

Russian Denial

Russia’s defence ministry denied that Russian missiles hit Polish territory, describing such reports as “a deliberate provocation aimed at escalating the situation”.

Kremlin spokesperson Dmitry Peskov said he had no information on an explosion in Poland.

Latvian Deputy Prime Minister Artis Pabriks said the situation was “unacceptable” and it could lead to NATO providing more anti-aircraft defences to Poland and Ukraine.

Lithuanian President Gitanas Nauseda said on Twitter: “Every inch of #NATO territory must be defended!”

Ukrainian Defence Minister Oleksiy Reznikov said Kyiv had warned of the danger Russian missiles posed to neighbouring countries and called for a no-fly zone to be imposed.

“We were asking to close the sky, because sky has no borders. Not for uncontrolled missiles. Not for the threat they carry for our EU & NATO neighbours. Gloves are off. Time to win,” he said in a Twitter post.