It comes after US President Joe Biden said it was "unlikely" the missile that killed two people in Nato-ally Poland was fired from Russia, but pledged support for Poland's investigation into what it had called a "Russian-made" missile.
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 on Tuesday. The officials spoke on condition of anonymity.
Biden's comments came after he convened an "emergency" meeting of the G7 and Nato leaders in Indonesia early on Wednesday morning for consultations on the attack 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."
The president, who was awakened overnight by staff with the news of the missile explosion while in Indonesia for the Group of 20 summit, earlier called Polish President Andrzej Duda to express his "deep condolences" for the loss of life.
Biden promised on Twitter "full US support for and assistance with Poland's investigation", and "reaffirmed the United States' ironclad commitment to NATO".
He 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 attack.
"I'm going to make sure we figure out exactly what happened," Biden said. "Then we're going to figure out our next step."
Meeting at a large round table in a ballroom in his hotel, Biden hosted the leaders of the G7, which includes Canada, France, Germany, Italy, Japan, the UK and the European Union, along with the president of the European Council and the prime ministers of Nato allies Spain and the Netherlands.
A statement from the Polish Foreign Ministry identified the missile as being made in Russia. But Poland's president 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.
The foundation of the Nato alliance is the principle that an attack against one member is an attack on them all.
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