Poland said a Russian-made missile fell in the country’s east, killing two people, though US President Joe Biden said it was “unlikely” it was fired from Russia.

The blast, which Ukrainian President Volodymr Zelenskyy decried as “a very significant escalation,” prompted Biden to call an emergency meeting of G-7 and NATO leaders. A deliberate, hostile attack on NATO member Poland could trigger a collective military response by the alliance.

But key questions around the circumstances of the missile launch remained amid the confusion caused by a blistering series of Russian airstrikes across the nearby border in Ukraine, none larger than who fired it. Russia denied any involvement in the Poland blast.

Three US officials said preliminary assessments suggested the missile was fired by Ukrainian forces at an incoming Russian one amid the crushing salvo against Ukraine’s electrical infrastructure Tuesday. The officials spoke on condition of anonymity because they were not authorised to discuss the matter publicly.

That assessment and Biden’s comments at the Group of 20 summit in Indonesia contradict information earlier Tuesday from a senior US intelligence official who told the AP that Russian missiles crossed into Poland.

The Polish government said it was investigating and raising its level of military preparedness. Biden pledged support for Poland’s investigation.