World leaders gathering at the G20 summit in Bali are attempting to diffuse a potential escalation in the months-long Ukraine war after a “Russian-made” missile struck NATO-member Poland killing two people.

The missile landed outside the rural Polish village of Przewodow, about four miles (6.4 kilometers) west from the Ukrainian border on Tuesday afternoon, roughly the same time as Russia launched its biggest wave of missile attacks on Ukrainian cities in more than a month.

The circumstances surrounding the incident, which marks the first time a NATO country has been directly hit during the almost nine-month conflict, remain unclear. It is not known who fired the missile, or precisely where it was fired from, though the Polish Foreign Ministry has described it as “Russian-made.”

Both Russian and Ukrainian forces have used Russian-made munitions during the conflict, with Ukraine deploying Russian-made missiles as part of its air defense system.

Speaking to reporters after holding an emergency meeting with G7 and NATO leaders on the sidelines of the G20 summit, US President Joe Biden said preliminary information suggested it was “unlikely” the missile was fired from within Russia, but was unable to say conclusively until the investigation was complete.

“We agreed to support Poland’s investigation into the explosion … And I’m going to make sure we figure out exactly what happened,” Biden said, adding the leaders offered sympathy over the death of two people. “Then we’re going to collectively determine our next step as we investigate and proceed. There was total unanimity among the folks at the table,” he added.

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Following Biden’s statement, a NATO military official told CNN the missile had been tracked by an alliance aircraft flying above Polish airspace at the time of the blast.

“Intel with the radar tracks [of the missile] was provided to NATO and Poland,” the NATO military official added. The NATO official did not say who launched the missile, or where it was fired from.

The missile strike within Poland’s borders on Tuesday “would not have happened without Russia’s horrific missile attacks against Ukraine,” Dutch Prime Minister Mark Rutte said in a statement online Wednesday.

“The G7 and present NATO members convened a meeting this morning in Bali during the G20 to discuss the incident in Poland last night. We are united in our message that we first need to establish the facts and therefore support Poland’s investigation,” Rutte wrote.

In comments earlier Tuesday, Polish President Andrzej Duda noted that while it was not clear who launched the missile, it was “most likely” made in Russia. “We are working calmly and in a very calm manner,” Duda said during an address from the Bureau of National Security in Warsaw.

The Kremlin has denied involvement in the explosion, with Russia’s Defense Ministry calling the reports by Polish media, who first reported the deaths,”a deliberate provocation in order to escalate the situation,” according to a short statement late Tuesday.

It added that the photos of wreckage published by Polish media “from the scene in the village of Przewodow have nothing to do with Russian weapons.”

The Russian mission at the United Nations on Wednesday said “the incident in Poland is an attempt to provoke a direct military clash between NATO and Russia,” adding the incident would be the focus of attention at the UN Security Council meeting later in the day.

Polish Prime Minister Mateusz Morawiecki said Tuesday that evidence suggests the missile that landed in Przewodów was a “single act” and there is no evidence of further missile strikes.

But while urging calm, Morawiecki said Poland would increase its military readiness and was contemplating the activation of Article 4 of the NATO Treaty. Article 4 is a consultation method that allows members of the 30-country alliance to bring an issue – usually about security – for discussion at the North Atlantic Council, its decision-making body.

Whatever the outcome of the Polish-led investigation, the incident has reinforced longstanding concerns related to the risk of battlefield miscalculation triggering NATO-Russian conflict.

Witness to the blast described hearing a terrifying “whoosh” as the projectile flew over the town and the force of the explosion shook nearby windows.

Video taken by a resident, which was geolocated and confirmed by CNN, shows a large smoke plume in the center of the village.

At the site of the explosion, local media showed an image of a crater and upturned farm vehicle. CNN cannot independently confirm the photos.

In his address, Duda said the US would send experts to investigate the site as part of joint operation.

Speaking after a call with Duda Tuesday, NATO Secretary General Jens Stoltenberg said it was “important that all facts are established.”

“I offered my condolences for the loss of life. NATO is monitoring the situation and Allies are closely consulting. Important that all facts are established,” said Stoltenberg in a statement. (CNN)

PHOTO CAPTION:•A crater is seen near the small village of Przewodów, Poland, where Polish officials confirmed that two people were killed after an explosion.

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