US President Joe Biden has told allies that a missile that killed two people in Poland was a Ukrainian air defence missile, a NATO source says.
Earlier, Biden said publicly that the missile was unlikely to have been fired from Russia.
If confirmed, that would likely alleviate concern that the first deadly incident in a NATO country since the war in Ukraine began might lead to escalation.
NATO ambassadors were due to hold an emergency meeting on Wednesday to discuss Tuesday's explosion at a grain dryer in eastern Poland near the Ukrainian border, which occurred while Russia was firing scores of missiles at cities across Ukraine.
Kyiv says it shot down most of the incoming Russian missiles with its own air defence missiles. Ukraine's Volyn region, just across the border from Poland, was one of the many Ukraine says was targeted by Russia's attacks.
The Russian Defence Ministry said none of its missiles struck closer than 35km from the Polish border, and that photos of the wreckage showed elements of a Ukrainian S-300 air defence missile.
Asked whether it was too early to say if the missile was fired from Russia, Biden said: "There is preliminary information that contests that. I don't want to say that until we completely investigate it, but it is unlikely in the lines of the trajectory that it was fired from Russia, but we'll see."
The United States and NATO countries would fully investigate before acting, Biden said in Indonesia after meeting other Western leaders on the sidelines of a summit of the G20 big economies.
The Kremlin said on Wednesday that some countries had made "baseless statements" about the incident, but that Washington had been comparatively restrained. Spokesman Dmitry Peskov told reporters that Russia had nothing to do with the incident, which he said had been caused by an S-300 air defence system.
The Polish Foreign Ministry said the rocket fell on Przewodow, a village about 6km from the Ukrainian border.
Polish President Andrzej Duda told reporters it was "most likely a Russian-made missile", but that there was no concrete evidence of who fired it. Both Russia's long-range missiles and air defence missiles used by Ukraine use Soviet designs.
A resident who declined to be identified said the two victims were men who were near the weighing area of a grain facility.
Some Western leaders said that whoever fired the missile, Russia was ultimately responsible.
"They stressed that, whatever the outcome of that investigation, Putin's invasion of Ukraine is squarely to blame for the ongoing violence," British Prime Minister Rishi Sunak's office said after a meeting with Canadian Prime Minister Justin Trudeau.