Biden spoke after he convened an “emergency” meeting of the Group of Seven and NATO leaders in Indonesia Wednesday morning for consultations on the explosion that killed two people in the eastern part of Poland near the Ukraine border.

“There is preliminary information that contests that,” Biden told reporters when asked if the missile had been fired from Russia. “It is unlikely in the lines of the trajectory that it was fired from Russia, but we’ll see.”

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

Ukraine still maintains stocks of former Soviet and Russian-made weaponry, including the S-300 air-defense missile system.

The president, who was awakened overnight by staff with the news of the missile explosion while in Indonesia for the Group of 20 summit, called Polish President Andrzej Duda early Wednesday to express his “deep condolences” for the loss of life. Biden promised on Twitter “full U.S support for and assistance with Poland’s investigation,” and “reaffirmed the United States’ ironclad commitment to NATO.”

Biden said that he briefed the allies on his conversations with Duda and NATO Secretary General Jens Stoltenberg and that there was “Total unanimity among the folks at the table” to support Poland’s investigation into the explosion.

“I’m going to make sure we find out exactly what happened,” Biden said. “And then we’re going to collectively determine our next step as we investigate.”

Meeting at a large round table in a ballroom in his hotel, the U.S. president hosted the leaders of the G-7, which includes Canada, France, Germany, Italy, Japan, the United Kingdom and the European Union, along with the president of the European Council and the prime ministers of NATO allies Spain and the Netherlands.

Biden held a separate meeting later with new British Prime Minister Rishi Sunak, in their first extended conversation since he took office last month.

“We’re going to continue to support Ukraine as long as Russia continues their aggression,” Biden said alongside Sunak, adding that he was “glad we’re on the same page” in backing Ukraine.

A statement from the Polish Foreign Ministry identified the missile as being made in Russia. But Poland’s president, Duda, was more cautious about its origin, saying that officials did not know for sure who fired it or where it was made. He said it was “most probably” Russian-made, but that is being still verified. If confirmed, it would be the first time since the invasion of Ukraine that a Russian weapon came down on a NATO country.

Biden also said the leaders condemned “the latest series of Russian missile attacks,” referring to the confirmed Russian strikes in recent days that have targeted Ukraine’s power grid and caused widespread blackouts.

“The moment when the world came together at the G-20 to urge de-escalation, Russia continues to escalate in Ukraine, while we’re meeting,” Biden said. “There were scores and scores of missile attacks in western Ukraine.”

Biden and his allies had set out to isolate Russia at the G-20 summit and the group’s final communique was expected to show that “most” of the nations in the G-20 condemn Russia’s invasion.

Underscoring the extraordinarily tense situation, CIA Director William Burns was in Kyiv on Tuesday during the barrage of Russian missiles. He remained in the U.S. embassy in Kyiv during the strikes.

Burns briefed Zelenskyy and Ukrainian officials on his meeting in Turkey with the head of Russia’s foreign intelligence service, according to another U.S. official who spoke on condition of anonymity to discuss intelligence matters.

Burns told Ukrainian officials that he had delivered a U.S. warning to Russia not to use nuclear weapons, the U.S. official said.

In addition to Biden’s conversations, Secretary of State Antony Blinken held talks with his Polish and Ukrainian counterparts Wednesday, the State Department said.

Army Gen. Mark Milley spoke with Polish Chief of the General Staff Gen. Rajmund Andrzejczak on Tuesday. The Joint Staff said the two generals discussed the explosion in Poland and the loss of life there. He provided no other details.