Poland's military was on high alert Wednesday after a deadly missile strike on a village near the border with war-ravaged Ukraine.
In Indonesia's Bali, Western leaders held an "emergency roundtable" on the sidelines of the G20 summit, where they urged against jumping to any conclusions about the origins of the strike.
The talks came after Poland's President Andrzej Duda said there was no clear evidence of who fired the missile that killed two people in the southeastern village of Przewodow, near the border with Ukraine.
He also said the missile was "most probably Russian-made".
An AFP journalist in Przewodow said police had cordoned off the blast site with sirens wailing in the distance.
US President Joe Biden said it was "unlikely" the missile had been fired from Russia, while France urged "utmost caution" in identifying who was behind the blast.
Moscow's ambassador has been summoned to provide "immediate detailed explanations" and the military had been put on heightened alert after an emergency national security council meeting, Polish authorities said.
"There has been a decision to raise the state of readiness of some combat units and other uniformed services," spokesman Piotr Muller told reporters after the meeting in Warsaw, adding that "our services are on the ground at the moment working out what happened".
Biden spoke by phone with Duda, offering "full US support for and assistance with Poland's investigation", the White House said.
The two leaders agreed to "remain in close touch to determine appropriate next steps as the investigation proceeds", it added.
- NATO commitment -
British Prime Minister Rishi Sunak, German Chancellor Olaf Scholz and French President Emmanuel Macron -- all leaders of NATO member states -- expressed solidarity with Poland.
Poland is protected by NATO's commitment to collective defence -- enshrined in Article 5 of its founding treaty -- but the alliance's response will likely be heavily influenced by whether the incident was accidental or intentional.
Biden also spoke with NATO chief Jens Stoltenberg about the incident, while ambassadors from the alliance were to hold an emergency meeting on Wednesday.
European Union chief Charles Michel said he was "shocked", and US Secretary of State Antony Blinken pledged to "remain closely coordinated in the days ahead as the investigation proceeds and we determine appropriate next steps".
- 'Significant escalation' -
Ukrainian President Volodymyr Zelensky had earlier said two Russian missiles hit Poland in what he described as "a very significant escalation".
Ukraine's Foreign Minister Dmytro Kuleba rejected as a "conspiracy theory" the idea that the Poland blast may have been caused by a surface-to-air missile fired by Kyiv's forces.
Russia's defence ministry meanwhile dismissed reports that it was to blame as a "provocation" intended to escalate tensions.
The explosion came after Russian missiles hit cities across Ukraine on Tuesday, including Lviv, near the border with Poland.
Zelensky said the strikes cut power to some 10 million people, though it was later restored to eight million of them, and also triggered automatic shutdowns at two nuclear power plants.
He said Russia had fired 85 missiles at energy facilities across the country, condemning the strikes as an "act of genocide" and a "cynical slap in the face" of the G20.
Moldova, which also borders Ukraine, reported power cuts because of the missiles fired at its neighbour and called on Moscow to "stop the destruction now".
- 'Now is the time' -
Zelensky told the G20 summit in Bali on Tuesday that "now is the time" to end the war.
In their final communique Wednesday, G20 members said they agreed the war hurts the global economy and warned against the threat or use of nuclear weapons in the conflict.
"Most" of the world's 20 biggest economies also condemned the war in Ukraine, the G20 nations said.
Russia's Vladimir Putin had decided to skip the summit, as he deals with the fallout from a string of embarrassing battlefield defeats in a war that his supporters believed would be over in days.
White House National Security Advisor Jake Sullivan said Russia was again trying to destroy Ukrainian critical infrastructure.
Since September, Ukraine forces have been pushing deeper into the south. Russia last week announced a full withdrawal from the regional capital of the southern Kherson region, allowing Ukraine's forces to re-enter the city.
Tuesday's missile strikes came after Russia-appointed officials in Nova Kakhovka said they were leaving the important southern city, blaming artillery fire from Kyiv's forces.
They also claimed "thousands of residents" had followed their recommendation to leave to "save themselves", saying Kyiv's forces would seek "revenge on collaborators".