The president of Poland said his government doesn't yet conclusively know who fired a missile that struck Polish territory late Tuesday, killing two civilians.
The explosion, which took place about 15 miles (24 km) from the Ukrainian border, came as Russia was launching a massive wave of missiles at Ukrainian cities and energy facilities just before dusk local time.
President Andrzej Duda said the projectile was "Russian-made" and variously referred to it as either a missile or a rocket.
"What happened was an isolated incident," said Duda, who added that an investigation is underway. "There is no indication that more will take place."
Poland is a member of NATO. The development set off a chain of diplomatic activity among members of the alliance and Ukraine, which is not a NATO member but which gets massive military aid from the alliance.
U.S. President Joe Biden spoke with Duda and "offered full U.S. support for an assistance with Poland's investigation," the White House said. "President Biden reaffirmed the United States' ironclad commitment to NATO."
Warsaw also was in communication with NATO Secretary General Jens Stoltenberg, U.K. Prime Minister Rishi Sunak, French President Emmanuel Macron and German Chancellor Olaf Scholz.
"The North Atlantic Alliance is on standby," Duda said. "We have strengthened the readiness of the Polish armed forces, including the air defense. Our planes will be supported by allied planes. We act calmly and prudently."
Russia's Ministry of Defense did not immediately respond to a CNBC request for comment but said in a Telegram post that Polish claims were "a deliberate provocation in order to escalate the situation."
Duda said Ukrainian President Volodomyr Zelenskyy had assured Warsaw that "it was a rocket launched from the territory of the Russian Federation."
Polish Minister of Foreign Affairs Zbigniew Rau summoned the ambassador of the Russian Federation "and demanded immediate detailed explanations," the Polish ministry said.
Leaders discuss Poland at G-20
Biden, China's Xi Jinping and other global leaders are gathered in Bali, Indonesia, for the Group of 20 summit. The Russian strikes began just hours after Zelenskyy finished giving a video speech to world leaders at the meeting where he outlined 10 demands that must be met before hostilities can cease in Ukraine.
Following an emergency meeting of national leaders to discuss overnight events in Poland, Biden acknowledged to reporters that details around the situation remained unclear.
"I'm going to make sure we figure out exactly what happened," Biden said. "And then we're going to collectively determine our next step as we investigate and proceed. There was total unanimity among the folks at the table."
G-20 nations are expected to issue a communique condemning Russia's invasion of Ukraine, according to a draft version of the document seen by CNBC.
The Kremlin has established a pattern of intensifying missile attacks on civilian targets in Ukraine whenever Russian troops suffer setbacks on the battlefield. Last week, Ukraine's army forced Russian invaders to flee Kherson, the only Ukrainian regional capital Moscow had managed to capture in nine months of brutal warfare.
NATO issued a joint statement with G-7 leaders that offered support to Poland and, separately, the Ukrainian people as they cope with another set of long-range attacks from the Russians.
"We condemn the barbaric missile attacks that Russia perpetrated on Ukrainian cities and civilian infrastructure Tuesday," the statement said.
'Harden the resolve'
Mick Ryan, an adjunct fellow at U.S.-based national security research organization CSIS, said in a Twitter post that it's unlikely Russia or NATO would use the strikes in Poland as a pretext for widening the war.
However, he said it's likely that NATO will now strengthen its air defenses along the borders with Russia, including Poland and the Baltic states.
"The attacks in Poland and Ukraine today will also harden the resolve of President Zelensky & Ukrainian people to achieve the ten elements of war termination that Zelensky outlined" at G-20, he said.
Ryan speculated that Poland will step up military assistance to Ukraine further, even if it's not yet clear how the missile ended up exploding on Polish territory.
"But regardless of whose missile this was," Ryan said, "it would not have fallen here if Russia had not invaded Ukraine."
U.S. futures turned lower overnight, and Asia-Pacific markets fell on Wednesday.
— NBC News' Ewa Galica and CNBC's Silvia Amaro contributed to this report.
