A missile that hit Poland killing two people was probably a Ukrainian air defence missile and there was no evidence to suggest the incident was an intentional attack by Russia, Poland's President Andrzej Duda said on Wednesday.
The announcement, which followed similar suggestions by the United States, was likely to ease global concern that the war in Ukraine could spill across the border.
NATO ambassadors were holding emergency talks to respond to the blast on Tuesday that killed two people at a grain facility in Poland near the Ukrainian border, the war's first deadly spillover onto the territory of the Western military alliance.
"From the information that we and our allies have, it was an S-300 rocket made in the Soviet Union, an old rocket and there is no evidence that it was launched by the Russian side," Poland's Duda said. "It is highly probable that it was fired by Ukrainian anti-aircraft defense."
Polish Prime Minister Prime Minister Mateusz Morawiecki said Warsaw might not need to activate Article 4 of the alliance's treaty, which calls for consultations when a country considers its security under threat.
Earlier, U.S. President Joe Biden said publicly that the missile was unlikely to have been fired from Russia. A NATO source said Biden had told allies that the missile was a Ukrainian air defence missile, and a Western diplomat confirmed that this was now the prevalent theory.
The incident occurred while Russia was firing scores of missiles at cities across Ukraine, in what Ukraine says was the biggest volley of such strikes of the nine-month war.
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 countrywide attacks.
The Russian Defence Ministry said none of its missiles had struck closer than 35 km (20 miles) from the Polish border, and that photos of the wreckage in Poland 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.
In a tweet issued hours after the incident, Ukraine's President Volodymyr Zelenskiy blamed it on "Russian missile terror". There was no immediate Ukrainian response on Wednesday to the reports that Washington now suspected it was in fact a stray Ukrainian missile.