Warsaw (AFP) - Poland put its military on high alert Tuesday after what the country's president said was "most probably" a strike by a Russian-made missile.

Western leaders held an "emergency roundtable" on Wednesday on the sidelines of the G20 summit in Indonesia, where they urged against jumping to any conclusions about the origins of the strike.

Warsaw said the missile had killed two people in the southeastern village of Przewodow, near the border with Ukraine, but did not have conclusive evidence of who fired it.

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 his Polish counterpart Andrzej 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 blast in Poland, 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, while Russia's defence ministry 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, while Washington said the Russian strikes in Ukraine would "deepen the concerns among the G20 about the destabilising impact of Putin's war".

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".