BRUSSELS — A top NATO official and Poland’s president said on Wednesday that a Ukrainian air-defense missile, not a Russian weapon, had most likely caused a deadly explosion on Polish territory, easing fears that the military alliance would become more deeply embroiled in the war.
Both men took pains to say that Ukraine was not to blame, noting that it had been fending off a major Russian aerial assault when the missile struck a Polish grain plant just over the border on Tuesday, killing two people.
“Let me be clear: This is not Ukraine’s fault,” said Jens Stoltenberg, the NATO secretary general. “Russia bears ultimate responsibility as it continues its illegal war against Ukraine.”
Officials said that what had struck the grain-processing plant was an S-300 missile, a munition that was built for air defenses but that had also been used by Russia to attack Ukraine; Poland’s justice minister said remnants of an S-300 had been found at the site of the explosion. Both Russia and Ukraine possess the systems — the earliest versions were developed in the Soviet Union — and that may have contributed to early confusion over who was responsible for the blast.
Poland’s president, Andrzej Duda, called the missile strike an “unfortunate accident” and said: “We have no evidence at the moment that it was a rocket launched by Russian forces. However, there are many indications that it was a missile that was used by Ukraine’s antimissile defense.”
Ukraine’s president, Volodymyr Zelensky, said his country bore no blame. “I have no doubt that it was not our rocket,” he told reporters in Kyiv, according to the Interfax news agency. Mr. Zelensky has appeared intent on capitalizing on the episode; on Tuesday, alluding to Poland’s membership in NATO, he accused Russia of an “attack on collective security.”
In the hours after the explosion in the village of Przewodow, about four miles from the Ukrainian border, top Polish officials said they were leaning toward invoking Article 4 of the NATO charter, under which members confer when a nation’s territorial integrity or security has been threatened.
But there was little talk of that on Wednesday. After a night of intense discussion, confusion and fear about whether Russia had somehow attacked a NATO ally, the results of the investigation so far seemed to lower the temperature.
After meeting with the alliance’s envoys in Brussels, Mr. Stoltenberg stressed that there was no indication of a deliberate attack by Russia or of any Russian plans to attack a NATO member, meaning that the alliance’s commitment to collective defense was not at issue. U.S. officials said they had accepted the preliminary conclusions of Poland and NATO.
Neither Russia nor NATO wants to go to war with the other. Even as NATO members have sent military aid to Ukrainian forces, both sides have shown considerable restraint in trying to avoid direct confrontation and keep the war from spilling over into neighboring countries.
Russia has denied responsibility for the explosion, saying that it did not aim any missiles near the border with Poland on Tuesday. On Wednesday, it appeared content with the statements coming out of the West.
“One should never rush to pronounce judgments and statements that can escalate the situation, still more so at such crucial moments,” said Dmitri S. Peskov, the Kremlin’s spokesman. “No hasty statements should be made before receiving accurate information.”
Mr. Peskov even offered some praise for a Russian adversary.
“In this case, it makes sense to pay attention to the restrained and far more professional response of the American side and the American president,” he said. President Biden said early Wednesday after an emergency meeting of allies in Indonesia that preliminary evidence showed the missile had not been fired from Russia.
Still, critical details about what actually transpired have yet to be clarified. There are questions about the trajectory of the missile that hit the grain-processing plant and whether it might have been aiming at — or had hit — a Russian missile in flight.
Mr. Duda said investigators had found no sign that a warhead detonated when the missile came down, suggesting that the blast had involved “explosion of the fuel that remained.”
The explosion occurred as Ukrainian military forces were responding to one of the biggest Russian missile barrages since the war began in February. Much of the attack was aimed at crippling Ukraine’s energy infrastructure, and on Wednesday it was clear that Moscow had done significant damage.
“Burned residential buildings, destroyed power plants again, hundreds of cities were left without electricity, water and heat,” Mr. Zelensky said in a virtual address to the Group of 20 summit in Indonesia on Wednesday. “Internet traffic has fallen by two-thirds — imagine the scale.”
The aerial assault left millions of people without power, compromised the connection of two nuclear plants to Ukraine’s national grid and prompted the national energy utility to impose sweeping blackouts.
In the western Lviv region, to which millions of people have moved in an effort to escape the fighting, central heating was suspended and there was no hot water in the main city as of Wednesday morning. In Kryvyi Rih, in central Ukraine, engineers at pumping stations were relying on large generators to keep the water flowing.
Gen. Mark A. Milley, chairman of the U.S. Joint Chiefs of Staff, denounced the barrage of up to 100 missiles fired on Tuesday at civilian infrastructure. “The deliberate targeting of the civilian power grid, causing excessive collateral damage and unnecessary suffering on the civilian population, is a war crime,” he said at a Pentagon news briefing.
General Milley said that Russia, in his view, could not achieve its military goals. The probability of Russia conquering Ukraine, he said, is “close to zero.”
But he said that Ukraine would find it difficult to achieve outright victory, as defined by driving Russia out of all of its territory, including Crimea, in the near future. Russia still has significant combat power in Ukraine, he said, despite its military setbacks.
Defense Secretary Lloyd J. Austin III, standing beside General Milley, demurred when asked if he shared that assessment, hinting at continued disagreement within the Biden administration about the course of the war.
“There are countless numbers of people that have been amazed and astonished by what the Ukrainians have accomplished,” said Mr. Austin, a former four-star Army general. “So I won’t presuppose what’s possible or impossible for them.”
In Bali, as the G20 meeting wound up on Wednesday, world leaders struggled to find common ground on the war in Ukraine in their closing statement, making clear the gulf between Western nations and other countries on Russia’s actions.
The summit did not result in the customary joint communiqué, but the officials agreed on a “leaders’ declaration.” The statement said that “most members strongly condemned the war in Ukraine,” but that “there were other views and different assessments of the situation and sanctions.”
Major powers like India and China have been unwilling to join the Western-led sanctions against Moscow, and Indonesia’s president, Joko Widodo, the chairman of this year’s summit, described the paragraph on the war as the most “highly debated.”
Steven Erlanger reported from Brussels, and Marc Santora from Kyiv, Ukraine. Reporting was contributed by Lara Jakes from Rome, John Ismay from Washington, Matthew Mpoke Bigg from London, Sui-Lee Wee from Bali, Indonesia, and Carly Olson in New York.