WARSAW: A missile that hit Poland was probably a stray fired by Ukraine’s air defences and not a Russian strike, Poland and NATO said on Wednesday, easing global concern that the war in Ukraine could spill across the border.

Nevertheless, NATO’s chief said that Moscow, not Kyiv was ultimately to blame, for starting the war in the first place and launching the attack that triggered Ukraine’s defences.

“This is not Ukraine’s fault. Russia bears ultimate responsibility as it continues its illegal war against Ukraine,” NATO Secretary-General Jens Stoltenberg told reporters in Brussels.

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,” Polish President Andrzej Duda said. “It is highly probable that it was fired by Ukrainian anti-aircraft defense.”

Stoltenberg also said it was likely a Ukrainian air defence missile.

Polish Prime Minister Prime Minister Mateusz Morawiecki said Warsaw might not need to activate Article 4 of NATO’s treaty, which calls for consultations when a member country considers its security under threat.

Earlier, U.S. President Joe Biden said the missile was unlikely to have been fired from Russia.

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 suggestions that it was a Ukrainian stray.

The missile fell on Przewodow, a village about 6 km (4 miles) from the Ukrainian border. 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 suggested that whoever fired the missile, Russia and President Vladimir Putin would ultimately be held responsible for an incident arising from its invasion.

“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 between Sunak and Canadian Prime Minister Justin Trudeau on the summit sidelines.

Leaders at the G20 summit issued a declaration saying “most members strongly condemned the war in Ukraine”, although it acknowledged that “there were other views and different assessments of the situation and sanctions”.

Russia is a member of the G20 and Ukraine isn’t, but Zelenskiy addressed the summit by video link, while Putin stayed home.

Moscow launched Tuesday’s wave of missile attacks just days after abandoning the southern city of Kherson, the only regional capital it had captured since the invasion.