US President Joe Biden called an “emergency” meeting of G7 and NATO leaders in Indonesia after NATO-ally Poland said a Russian-made missile killed two people in the eastern part of its country near the Ukraine border.

Biden, who was awakened overnight by staff with the news of the missile explosion, called Polish President Andrzej Duda to express his “deep condolences” for the loss of life. Biden promised “full US support for and assistance with Poland’s investigation,” and “reaffirmed the United States’ ironclad commitment to NATO”.

A statement from the Polish Foreign Ministry identified the missile as being made in Russia. But Poland's president Duda was more cautious about its origin, saying that officials did not know for sure who fired it or where it was made.

He said it was “most probably” Russian-made, but that is being still verified. If confirmed, it would be the first time since the invasion of Ukraine that a Russian weapon came down on a NATO country.

The scene of an explosion in Przewodow, a village in eastern Poland. – Reuters

Leaders from the United States, Germany, Canada, Netherlands, Japan, Spain, Italy, France and the United Kingdom were taking part in the meeting. All except for Japan are members of NATO.

The leaders were briefly seen together around a conference table at the start of the meeting. Biden said "no" when reporters asked whether he could share what he knows about the explosion. He did not comment when asked about what involvement Russia might have.

Poland summoned Russia's ambassador to Warsaw for an explanation after Moscow denied it was responsible.

The foundation of the NATO alliance is the principle that an attack against one member is an attack on them all.

Poland was increasing the readiness of some military units and determining whether to request consultations with NATO allies.

"A moment ago we decided to verify whether there are grounds to launch procedures under Article 4 of the North Atlantic Treaty," Polish government spokesperson Piotr Muller said.

Article 4 allows NATO members to bring any issue of concern, especially regarding security, for discussion at the North Atlantic Council.

Polish government spokesperson Piotr Muller speaks to the media. – Reuters

Muller also said Poland was increasing the readiness of some military units, but declined to take any question including about the cause of the incident.

"It was decided a moment ago to increase the readiness of some combat military units in Poland and to increase the combat readiness of other units of uniformed services in our country," he said.

The explosion occurred after Russia hammered cities across Ukraine with missiles, attacks that Kyiv said were the heaviest wave of strikes nearly nine months into the Russian invasion. Some hit the western city of Lviv, less than 80km (49.7 miles) from the border with Poland.

Poland is a member of the US-led Western military alliance NATO, which is committed to collective defence, and the possibility that the Polish explosion resulted from an intentional or accidental Russian strike raised alarm.

A picture on social media of the blast in Poland seen in the distance. – Reuters

A NATO official said the alliance was looking into the reports and closely coordinating with Poland.

The Associated Press cited a senior US intelligence official as saying the blast in the eastern Polish village of Przewodow was caused by Russian missiles crossing into Poland.

In Washington, the Pentagon and the US State Department said they could not confirm Russian missiles had landed on Polish territory.

"We are aware of the press reports alleging that two Russian missiles have struck a location inside Poland near the Ukraine border.

"I can tell you that we don't have any information at this time to corroborate those reports and are looking into this further," Pentagon spokesperson Brigadier General Patrick Ryder said.

Reports coming out of Poland 'incredibly concerning' says US State Department spokesperson Vedant Patel. – Reuters

Russia's defence ministry denied reports that Russian missiles had landed in Poland, describing them as "a deliberate provocation aimed at escalating the situation".

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

Muller told reporters the committee was convened due to the "crisis situation". 

Polish Radio ZET reported that two stray missiles hit Przewodow, killing two people, without elaborating. The village is 6km (3.5 miles) from the border with Ukraine.

"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.

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European Commission President Ursula von der Leyen said she was "alarmed" by reports of an explosion in Poland.

"We are closely monitoring the situation and are in touch with Polish authorities and partners and allies", she said on Twitter.

"I extend my condolences and my strongest message of support and solidarity with Poland and our Ukrainian friends."

Ukrainian President Volodymyr Zelensky said that "Russian missiles hit Poland".

Senior officials from Germany, Norway, Lithuania and Estonia – all NATO members – said they were trying to glean more information together with Poland and other allies.

"This is a very serious incident but much remains unclear," Norwegian Foreign Minister Anniken Huitfeldt said.

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

NATO was looking into the reports, an official said.

"We are looking into these reports and closely coordinating with our ally Poland," the NATO official said.

The European Union stood with Poland, the president of the European Council Charles Michel said in a tweet after reports of the blasts close to the Ukrainian border.

"I am in contact with Polish authorities, members of the European Council and other allies", he said.

US President Joe Biden talks on the phone to Polish President Andrzej Duda as security adviser Jake Sullivan and Secretary of State Antony Blinken listen. – AP 

NATO member Lithuania's president said that "every inch of NATO territory" must be defended.

"Concerning news from Poland tonight on at least two explosions. Keeping a close contact with our Polish friends. Lithuania stands in strong solidarity with Poland. Every inch of NATO territory must be defended," President Gitanas Nauseda tweeted.

"The cause of the explosions is not yet known, but at the same time Russian missiles attacked Ukraine," Nauseda added in a Facebook post. "The terrorist regime will be stopped by united force of the allies".

Germany is monitoring the situation in Poland closely and is in contact with Warsaw and its NATO allies, Foreign Minister Annalena Baerbock said.

"My thoughts are with #Poland, our close ally and neighbour," Baerbock tweeted.

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

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."

The US State Department earlier said Washington was working to determine what happened and was working with the Polish government.

Zelensky called it a "significant escalation" of the conflict.

"Russian missiles hit Poland, the territory of our friendly country. People died," Zelensky said in his nightly video address.

On Tuesday, Russia had launched more than 100 missile and drone attacks on Ukraine in the latest escalation of its invasion. Zelensky said that only about 10 hit their targets.

"The longer Russia feels impunity, the more threats there will be to anyone within reach of Russian missiles. To fire missiles at NATO territory. This is a Russian missile attack on collective security. This is a very significant escalation. We must act," he said.

"Today happened what we were warning about for a long time: We told that terror is not confined to our state borders.

"I now want to tell every one of our Polish brothers and sisters – Ukraine will always support you. Free people won't be cowed by terror. Victory is possible when there is no fear. And we and you are not afraid."

Air raid sirens blared and explosions rang out in nearly a dozen major Ukrainian cities, echoing a pattern in recent weeks of Moscow lashing out far from the front after losses on the battlefield, most recently the major southern city of Kherson.

Russian barrage brings darkness to Ukraine. – AP

Russia had launched 110 missiles and 10 Iranian-made attack drones into Ukraine by early evening, Ukraine's armed forces General Staff said in a statement.

Zelensky said the main target of the missile flurry was energy infrastructure, as before, though added that only 10 intended targets in all had been hit.

In the capital Kyiv, flames funnelled out of a five-storey apartment block after being hit by what residents said appeared to be shot-down pieces of missile. The emergency service said one person was confirmed killed and another injured. Kyiv's mayor said half the capital was left without electricity.

Other strikes or explosions were reported in cities ranging from Lviv and Zhytomyr in the west to Kryvy Rih in the south and Kharkiv in the east. Regional officials reported some of the attacks had knocked out electricity, water and heating.

The attacks had left millions of Ukrainians without energy in 16 of the country's 24 regions including Kyiv, the UN humanitarian office (OCHA) said in a statement.