The Associated Press earlier cited a senior United States intelligence official as saying the blast was due to Russian missiles crossing into Poland.

But the Pentagon said it could not confirm that account.

The White House also said it could not confirm reports coming out of Poland and was working with the Polish government to gather more information.

Russia’s defence ministry denied reports Russian missiles had hit Polish territory, describing them as “a deliberate provocation aimed at escalating the situation”.

It added in a statement: “No strikes on targets near the Ukrainian-Polish state border were made by Russian means of destruction.”

The Kremlin did not immediately respond to requests for comment.

Polish Prime Minister Mateusz Morawiecki has called an urgent meeting of a government committee for national security and defence affairs, government spokesman Piotr Muller said on Twitter.

Latvian Deputy Prime Minister Artis Pabriks said on Twitter Russia “fired missiles which target not only Ukrainian civilians but also landed on NATO territory in Poland”.

Officials from Norway, Lithuania and Estonia – members of the NATO defence alliance – said they were trying to find out more.

“This is a very serious incident but much remains unclear,” Norwegian Foreign Minister Anniken Huitfeldt said, according to Norwegian news agency NTB.

Members of the North Atlantic Treaty Organisation (NATO) are committed to collective defence and the possibility the Polish explosion resulted from an intentional or accidental Russian strike raised alarm.

“Every inch of #NATO territory must be defended!” Lithuanian President Gitanas Nauseda said on Twitter.

Estonian Foreign Minister Urmas Reinsalu said, according to BNS newswire: “We are discussing with our allies how to respond to what happened jointly and decisively.”

Polish Radio ZET reported earlier that two stray missiles hit Przewodow, killing two people, without giving any more details.

“Firefighters are on the spot. It’s not clear what has happened,” said Lukasz Kucy, an officer on duty at a firefighters’ post near the Polish village.

Russia was pounding cities across Ukraine with missiles on Tuesday, in attacks Kyiv said were the heaviest wave of missile strikes in almost nine months of war.

Some hit Lviv, which is less than 80km from the border with Poland.

Muller told reporters later on Tuesday the committee was convened due to the “crisis situation”, adding relevant information would be presented to the public later.

He called on media not to publish “unverified information” in the meantime.

Missiles rained down on Ukraine cities including the capital Kyiv, Lviv and Rivne in the west, Kharkiv in the northeast, Kryvyi Rih and Poltava in the centre, Odesa and Mikolaiv in the south and Zhytomyr in the north.

A body was pulled out of a residential building that was hit and set ablaze in central Kyiv, and a senior presidential official said the power situation was “critical” after heavy damage to energy infrastructure.

In a video posted online, President Volodymyr Zelenskiy warned Ukrainians that more missile strikes were possible but added: “We are working, we will restore everything, we will survive.”

Power was knocked out in areas of several cities including Kyiv, Lviv and Kharkiv, and the national grid operator announced emergency electricity outages in northern and central regions, and in Kyiv.

Kyiv’s mayor said half of the capital was without electricity. The mayor of Lviv said 80 per cent of the city had no electricity so lighting, water and heating supplies were off.

City authorities in Vinnytsia in west-central Ukraine were told to stock up on water following damage to a pumping station.

“This is the most massive shelling of the power system since the beginning of the war,” Energy Minister German Galushchenko said.

Air force spokesperson Yuiy Ihnat said more than 100 missiles had been fired at Ukraine, surpassing the 84 fired by Russia on October 10 in what was previously the heaviest air strikes.

The country’s biggest mobile phone provider warned of possible signal outages, and the transport system suffered disruptions in several areas.

In the capital, a five-storey apartment block was left smouldering after being hit by what residents said appeared to be shot-down missile parts.

Rescue workers and medics were quickly on the scene.

“I was in the apartment during the air raid warning. I saw a bright light in my window, and understood that something was coming. Then I heard the sound, as it was nearing,” said Oleksandra, 22, who lives in the apartment block.

“I saw from my window as the rocket was flying, a bright fire, and the sound of something flying very close by. I immediately went outside … I saw people were running out of our building and that there was smoke.

“The last flat I lived in was also hit,” she added. “Thankfully I was abroad at the time.”

The attacks followed days after a humiliating retreat by Russian forces from the southern city of Kherson and coincided with a summit of the G20 nations in Bali that was dominated by discussion of the war Ukraine.

“This is what Russia has to say on the issue of peace talks,” Foreign Minister Dmytro Kuleba wrote on Twitter.

“Stop proposing Ukraine to accept Russian ultimatums! This terror can only be stopped with the strength of our weapons & principles.”