A NATO spokesperson said Secretary General Jens Stoltenberg will chair an emergency meeting of alliance ambassadors Wednesday in Brussels to discuss the incident.

Poland’s foreign ministry said earlier Tuesday it had confirmed that the missile was Russian-made and that it had summoned the Russian ambassador “with a demand for an immediate detailed explanation,” according to a statement.

Polish President Andrzej Duda, who spoke with Biden, was less definitive, saying that the missile was “probably” Russian-made, cautioning: “We do not have conclusive evidence at this time of who launched the missile.” Ukraine’s military also uses some Russian-manufactured missiles.

Moscow and Kyiv quickly pointed fingers at each other.

Ukrainian President Volodymyr Zelenskyy, in a video message, said “Russian missiles hit Poland,” calling it an “attack on collective security” and “a very significant escalation.” Russia’s Foreign Ministry denied the charge, tweeting “Russian hardware has launched NO strikes at the area.”

Meanwhile, speculation swirled on social media — partly fueled by pro-Russian bots — that a Ukrainian air-defense missile launched to counter Russian weapons is what crashed in Poland. But Ukrainian Foreign Minister Dmytro Kuleba railed against that charge, calling it a “conspiracy theory” in a tweet.

The explosion in Poland came amid a barrage of Russian missiles fired at Ukrainian energy and electricity facilities across the country this week, plunging the capital of Kyiv and the recently recaptured city of Kherson into darkness. Ukrainian Defense Minister Oleksii Reznikov tweeted that 73 out of over 90 cruise missiles launched by Russia were shot down over the course of the evening, along with 10 more suicide drones.

“They’ve been totally unconscionable with what they’re doing,” Biden said after the emergency meeting in Bali. “And at a moment when the world is coming together at the G-20 to urge de-escalation, Russia continues to escalate in Ukraine.”

Senior U.S. officials have for more than a month discussed the possibility that the war in Ukraine would spill over into neighboring countries. Those officials, in coordination with European leaders, have conducted military planning exercises to game out such a scenario, according to a readout of a U.S. meeting with NATO officials in October obtained by POLITICO. The Pentagon did not immediately comment on the readout.

The next few hours and days after the initial report of the strike on Poland will be critical, as NATO members anxiously await the cause of the explosion. A mistake by Ukraine could be forgiven, experts say, but even an errant strike by Russia could prompt a more forceful response by officials throughout the alliance.

In Poland, Prime Minister Mateusz Morawiecki said after an emergency cabinet meeting that the country had decided to “raise the combat readiness of selected branches of the Polish Armed Forces, with particular emphasis on airspace monitoring.”

Soon after the first reports on Tuesday, Senate Foreign Relations Chair Bob Menendez (D-N.J.) said that if Moscow intentionally struck territory in Poland, it could lead to the invocation of Article 5 of the NATO charter. That provision calls for other NATO member-states to contribute to the response, including using military force, if one member is attacked.

“We’re drilling down to find out what were the circumstances of it. It’s obviously very important to understand — was it a mistake, was it an overflight, was it intentional. I hope that it was not intentional,” Menendez said in an interview.

“I hope that the Russians apologize quickly for the loss of life and express that it wasn’t intentional. Obviously, if it was intentional, that has all kinds of consequences to it,” he added. “It’s definitely an enlargement of the conflict and of course it brings into question Article 5.”

At the Pentagon, Press Secretary Brig. Gen. Patrick Ryder was asked about the United States’ security commitments. “We’ve been crystal clear that we will defend every inch of NATO territory,” he said.

Sen. Chris Coons (D-Del.), a top Biden ally, said media reports of the explosion were announced at the end of Senate Democrats’ weekly lunch on Tuesday, calling it a “stunning” development.

“I think it’s critical that the Russians promptly recognize this was a tragic mistake, apologize for it and offer compensation or this is going to quickly become a challenging issue,” Coons added.

Late Tuesday, Sens. Jeanne Shaheen (D-N.H.) and Thom Tillis (R-N.C.), co-chairs of the Senate NATO Observer Group, issued a statement reaffirming support for Poland and NATO allies.

European officials are also expressing their concern, and are blaming the Russians. “It is appalling to see a desperate regime attacking critical infrastructure of Ukraine and hitting allied territory with victims,” said a senior European diplomat who asked to be anonymous to discuss a fluid situation. “If confirmed, this represents another level of escalatory move by Russia against NATO. Poland is a very dear ally. NATO solidarity and support for Poland is ironclad.”

Since before Russia’s full-scale invasion of Ukraine on Feb. 24, Biden has been adamant that U.S. troops will not fight in any conflict in Ukrainian territory. He has also held back on providing certain types of weapons to the Ukrainians out of concern that they could be used to strike inside Russian territory and widen the war. Biden and other NATO leaders have repeatedly warned Russia that it must not target countries in the military alliance, and to date the Kremlin has been cautious about avoiding such direct conflict.

The attacks come a day before the next virtual meeting of the Ukraine Defense Contact Group, a collection of 50 countries that gather monthly to plan next steps for arming and supporting Ukraine.

The Biden administration Tuesday asked Congress for $37.7 billion in Ukraine funding as part of a supplemental package, with $21.7 billion of that going directly to arms for Kyiv and to restock U.S. warehouses after eight months of shipping ammunition and equipment to Ukraine. Another $7 billion will be given to the White House to draw down existing U.S. weapons and ammunition stocks to send to Ukraine quickly.

Lara Seligman, Erin Banco, Meredith Lee Hill, Paul McLeary, Jan Cienski, Lili Bayer and Nahal Toosi contributed to this report.