Western leaders and diplomats will hold a series of emergency meetings on Wednesday after a rocket landed on Polish territory amid a Russian bombardment of neighbouring Ukraine. Warsaw has put its troops on high alert and opened up consultations with NATO allies in the wake of the fatal incident.

In a statement issued on Tuesday afternoon, Poland’s Foreign Ministry said that “at 3.40pm today, a Russian-made missile landed in the village of Przewodów,” a rural farming community in the east of the country, just 10 kilometers from the border with Ukraine. According to officials, two Polish citizens were killed in the blast.

“We are acting with calm,” Polish President Andrzej Duda said, warning that “this is a difficult situation.” Warsaw has summoned the Russian ambassador to provide an explanation.

In response, envoys from NATO states, including Poland, will meet in Brussels to discuss a potential response, while investigators race to determine which side launched the missile and whether it amounts to a deliberate attack on the bloc. Such a strike could be grounds for triggering Article 5 of the alliance’s foundational treaty, which states that aggression against any one member is considered aggression against all 30 members.

However, speaking at the G20 summit of world leaders in Bali hours after Warsaw sounded the alarm, US President Joe Biden said it was “unlikely in the lines of trajectory that it was fired from Russia, but we’ll see.”

The initial findings of a probe into the circumstances has determined that the missile could have been launched by Ukrainian air defence forces as part of efforts to shoot down an incoming Russian rocket, Associated Press reported on Wednesday morning, citing unnamed American officials.

Kyiv issued air raid warnings across the entire country on both Tuesday and Wednesday after Russia launched as many as 85 missiles against civilian infrastructure, leaving several cities without power. Analysts had warned that the Kremlin could step up its bombing campaign after its troops were forced to retreat from the occupied city of Kherson last week as Ukraine continues to liberate territory as part of a major counteroffensive.

Russian missiles have previously violated the airspace of other nations in the region, with Moldova having lodged a protest after a rocket launched from a ship in the Black Sea crashed in the country after being shot down by Ukrainian air defences. Both Moscow and Kyiv’s forces operate the Soviet-designed S-300 surface-to-air launcher that can target warplanes and cruise missiles at a range of up to 40 kilometers.

In an address to his citizens on Tuesday night, Ukrainian President Volodymyr Zelenskyy said that the explosion in Przewodów was proof that “terror is not limited by our state borders.” According to him: “We need to put the terrorist in its place. The longer Russia feels impunity, the more threats there will be for everyone within the reach of Russian missiles.”