KYIV, Ukraine (AP) — Poland said early Wednesday that a Russian-made missile fell in the eastern part of the country, killing two people in a blast that Ukrainian President Volodymr Zelenskyy decried as “a very significant escalation” of the war.

Serious questions about the explosion remain, including who fired the missile. Russia denied any involvement.

U.S. President Joe Biden said it was “unlikely” that the missile was fired from Russia, but he convened an emergency meeting of the Group of Seven and NATO leaders who were in Indonesia for another summit. It was not immediately clear whether Biden was suggesting that the missile hadn’t been fired by Russia at all.

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

Zelenskyy’s comments, delivered in an evening address to the nation, came hours after a senior U.S. intelligence official told The Associated Press that Russian missiles had crossed into Polish territory and killed two people.

A second person said that apparent Russian missiles struck a site in Poland about 15 miles from the Ukrainian border.

A statement from the Polish Foreign Ministry identified the weapon as being made in Russia. President Andrzej Duda was more cautious, saying that it was “most probably” Russian-made but that its origins were still being verified.