Poland said early Wednesday morning that a Russian-made missile fell in the east of the country, killing two people, although U.S. President Joe Biden said it was unlikely to have been fired from Russia.

The explosion, which Ukrainian President Volodymyr Zelenskiy condemned 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 from the alliance.

But key questions about the circumstances surrounding the missile launch remained amid the confusion caused by a sharp series of Russian airstrikes on the nearby border in Ukraine. Russia has denied any involvement in the bombing in Poland.

Three U.S. officials told the Associated Press that preliminary estimates suggest a missile was fired by Ukrainian forces at an approaching Russian missile during a crushing salvo on Ukraine's electrical infrastructure Tuesday. The officials spoke on condition of anonymity because they are not authorized to discuss the matter publicly.

That assessment and Biden's comments at the G-20 summit in Indonesia contradict information obtained earlier Tuesday from a senior U.S. intelligence official who told the AP that Russian missiles had crossed into Poland.

The Polish government said it was investigating and increasing its military readiness. Biden promised to support Poland's investigation.

The Polish Foreign Ministry said in a statement that the weapons were made in Russia. President Andrzej Duda was more cautious, saying it was most likely produced in Russia, but its origin is still being verified.