KYIV, Ukraine -- Poland's President Andrzej Duda says there is no evidence a missile that hit Poland near its border with Ukraine was an "intentional attack."
Duda said Wednesday that the landing of the Russian-made missile in a rural area, killing two people, was mostly likely an accident.
"It was not an attack on Poland," Duda said, adding that Tuesday's incident involved "most probably a Russian-made missile."
"We have no proof at this point to suggest the missile was fired by the Russian side," Duda said.
He added: "There is high probability that it was a missile used for anti-missile defense, meaning it was used by Ukraine's defense forces."
"Ukraine's defense was launching their missiles in various directions and it is highly probable that one of these missiles unfortunately fell on Polish territory," Duda said.
Even so, he said the ultimate responsibility lies with Russia, which launched a barrage of missile attacks on Ukraine on Tuesday.
U.S. President Joe Biden said it was "unlikely" the missile was fired from Russia.
The blast, which Ukrainian President Volodymr Zelenskyy decried as "a very significant escalation," prompted Biden to call an emergency meeting of G-7 and NATO leaders. A deliberate, hostile attack on NATO member Poland could trigger a collective military response by the alliance.
But key questions around the circumstances of the missile launch - none larger than who fired it - remained amid the confusion caused by a blistering series of Russian airstrikes across the nearby border in Ukraine. Russia denied any involvement in the Poland blast.
Three U.S. 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 Tuesday. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.
That assessment and Biden's comments at the Group of 20 summit in Indonesia contradict information earlier Tuesday from a senior U.S. intelligence official who told the AP that Russian missiles crossed into Poland.
The Polish government said it was investigating and raising its level of military preparedness. Biden pledged support for Poland's investigation.
A statement from the Polish Foreign Ministry identified the weapon as being made in Russia.
"We are acting with calm," Duda said. "This is a difficult situation."
Ukraine still maintains stocks of former Soviet and Russian-made weaponry, including the S-300 air-defense missile system.
Biden's decision to convene the emergency meeting upended schedules for the final day of the Group of 20 summit in Indonesia.
European Commission President Ursula von Der Leyen said participants there discussed the blast in Poland and expressed their continued solidarity with Ukraine.
"We offer our full support to Poland and assistance with the ongoing investigation. We will remain in close contacts with our partners on the next steps. We will stand with Ukraine as long as it takes," she said.
Biden, who was awakened overnight by staff with the news of the missile while attending the summit, called Duda to express his condolences. On Twitter, Biden promised "full U.S support for and assistance with Poland's investigation," and "reaffirmed the United States' ironclad commitment to NATO."
NATO Secretary-General Jens Stoltenberg called the meeting of the alliance's envoys in Brussels. The U.N. Security Council also planned to meet Wednesday for a previously scheduled briefing on the situation in Ukraine.
Poland's statement did not address whether the strike could have been a targeting error or if the missile could have been knocked off course by Ukrainian defenses.
Poland and NATO used language that suggested they were not treating the missile blast as an intentional Russian attack, at least for now. A NATO statement called it a "tragic incident."
If Russia had deliberately targeted Poland, it would risk drawing the 30-nation alliance into the conflict at a time when it is already struggling to fend off Ukrainian forces.
The Kremlin is offering rare praise for the United States, applauding President Joe Biden's "restrained" reaction to reports about a Russian-made missile landing in Poland.
Kremlin spokesman Dmitry Peskov on Wednesday dismissed much of the reaction to the missile hit as "hysterical, frenzied."
Elsewhere, officials expressed alarm that the war could be escalating and spread to neighboring countries.
Peskov said that "high-ranking officials from different countries made statements without having any idea what happened exactly, what caused it, and so on."
Asked to comment on Tuesday's barrage of strikes on Ukrainian energy infrastructure, Peskov said that "objects that directly or indirectly have to do with military infrastructure" were targeted.
Polish media reported that the strike took place in an area where grain was drying in Przewodów, a village near the border with Ukraine.
CIA Director William Burns was in Kyiv on Tuesday during the barrage of Russian missiles that struck dozens of targets in the country. He remained in the U.S. Embassy in Kyiv during the strikes.
Burns briefed Zelenskyy and Ukrainian officials on his meeting in Turkey with the head of Russia's foreign intelligence service, according to another U.S. official who spoke on condition of anonymity to discuss intelligence matters.
Burns told Ukrainian officials that he had delivered a U.S. warning to Russia not to use nuclear weapons, the U.S. official said.
The Russian Defense Ministry denied being behind "any strikes on targets near the Ukrainian-Polish border" and said in a statement that photos of purported damage "have nothing to do" with Russian weapons.
Polish Foreign Minister Zbigniew Rau summoned the Russian ambassador and "demanded immediate detailed explanations," the government said.
The strike came to light as Russia pounded Ukraine's energy facilities with its biggest barrage of missiles yet, striking targets across the country and causing widespread blackouts.
The barrage also affected neighboring Moldova. It reported massive power outages after the strikes knocked out a key power line that supplies the small nation, an official said.
The missile strikes plunged much of Ukraine into darkness and drew defiance from Zelenskyy, who shook his fist and declared: "We will survive everything."
In his nightly address, the Ukrainian leader said the strike in Poland offered proof that "terror is not limited by our state borders."
Russia fired over 90 missiles and over 10 attack drones at Ukraine on Tuesday, Ukraine's General Staff said in its Wednesday morning update. Ukrainian forces shot down 77 missiles and 11 drones, it said.
The Ukrainian energy minister said the attack was "the most massive" bombardment of power facilities in the nearly 9-month-old invasion, striking both power generation and transmission systems.
The minister, Herman Halushchenko, accused Russia of "trying to cause maximum damage to our energy system on the eve of winter."
The assault killed at least one person in a residential building in the capital, Kyiv. It followed days of euphoria in Ukraine sparked by one of its biggest military successes - the retaking last week of the southern city of Kherson.
The power grid was already battered by previous attacks that destroyed an estimated 40% of the country's energy infrastructure.
With its battlefield losses mounting, Russia has increasingly resorted to targeting Ukraine's power grid, seemingly hoping to turn the approach of winter into a weapon by leaving people in the cold and dark.
In other developments, leaders of most of the world's economic powers were drawing closer to approval of a declaration strongly denouncing Russia's invasion.
On Tuesday, Biden and Zelenskyy pressed fellow G20 leaders at the summit in Indonesia for a robust condemnation of Russia's nuclear threats and food embargoes. More discussion and a possible vote were expected Wednesday.