There are fears of a major escalation of the Ukraine war after stray Russian missiles apparently crossed into Poland, killing two people.

Polish media are reporting an explosion near the village of Przewodów, close to the border with Ukraine, in an area where locals were drying grain.

Firefighters confirmed two people were dead but not the cause.

“Firefighters are on the spot, it’s not clear what has happened,” said Lukasz Kucy, officer on duty at a nearby firefighters’ post.

It came as Russia unleashed its heaviest barrage of the war to date after Ukraine President Volodymyr Zelensky pleaded with the “G19” to end the invasion.

In breaking news on Wednesday morning (Australian time), Polish Prime Minister Mateusz Morawiecki called an emergency meeting of the country’s top national security and defence officials.

Government spokesman Piotr Muller did not confirm the missile strike but said the meeting was about a “crisis situation”.

US media are quoting a senior US intelligence source who confirmed Russian missiles had crossed into Poland, which is a member of NATO.

However, the Pentagon said it could not confirm reports.

Russia’s defence ministry has denied the reports, saying they were a “deliberate provocation aimed at escalation”.

Russia’s worst attack

Earlier, up to 100 Russian missiles thundered down across Ukraine in what Kyiv said was the heaviest barrage of the war.

Moscow upped its attacks as world leaders considered a draft declaration condemning the war and after Russia’s humiliating retreat from the prized Kherson region.

It came after the Mr Zelensky deliberately snubbed Russia as he gave a video link address pointedly referring to the “G19” leaders gathered in Bali as he said it was the time for the world to stop the invasion.

Russia is represented at the G20 summit by Foreign Minister Sergei Lavrov in place of President Vladimir Putin.

Later, a storm of missiles slammed into the capital Kyiv, Lviv and Rivne in the west, said Ukraine.

Missiles also came down in Kharkiv in the north-east, Kryvyi Rih and Poltava in the centre, Odesa and Mikolaiv in the south and Zhytomyr in the north.

Mr Zelensky’s chief of staff said Russia had responded to Ukraine’s calls for peace with more aggression.

“Russia responds to Zelenskyy’s powerful speech at G20 with a new missile attack. Does anyone seriously think that the Kremlin really wants peace?” tweeted Andriy Yermak.

“It wants obedience. But at the end of the day, terrorists always lose.”

A residential building hit by a Russian missile strike in Kyiv. Photo: Getty

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.

In his address to the G20, Mr Zelensky said the world should act before Russia had time to regroup.

“We will not allow Russia to wait it out, build up its forces, and then start a new series of terror and global destabilisation,” he said via video link to Indonesia.

“I am convinced now is the time when the Russian destructive war must and can be stopped.”

‘We will survive’

The storm of missiles knocked out power in 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.

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

G20 condemns war

The G20’s draft declaration states that most members strongly condemn the war in Ukraine, which is exacerbating fragilities in the global economy.

The G20 members also voiced deep concern at the challenges posed to global food security, and called for central bank independence to rein in soaring inflation, the draft showed.

“Most members strongly condemned the war in Ukraine and stressed it is causing immense human suffering and exacerbating existing fragilities in the global economy,” the draft said, suggesting that Russia had opposed the language.

“There were other views and different assessments of the situation and sanctions,” said the draft declaration, which was confirmed by a European diplomat.

The 16-page document has yet to be adopted by G20 members.

Wrangling over how to describe the war in Ukraine prevented G20 ministers earlier this year from issuing a joint communique.

In the first session of the summit held on Tuesday, many countries condemned Russia’s invasion, according to one person who was present.

The summit, which host Indonesia and other countries have said should focus on risks to the global economy, has been overshadowed by the invasion of Ukraine.

“Recognising that the G20 is not the forum to resolve security issues, we acknowledge that security issues can have significant consequences for the global economy,” the draft declaration said.

Prisoner torture on both sides: UN

A UN monitoring team says it has found that both Russia and Ukraine tortured prisoners of war during the nearly nine-month conflict.

The UN human rights office (OHCHR) cited examples including the use of electric shocks and forced nudity.

The findings were based on interviews with more than 100 prisoners of war on each side of the conflict.

Matilda Bogner, head of the monitoring mission, told a Geneva press briefing that the “vast majority” of Ukrainian prisoners they interviewed held by Russian forces reported torture and ill-treatment.

She gave examples of dog attacks, electric shocks and sexual violence.

Russia, which invaded Ukraine on February 24, denies torture or other forms of maltreatment of POWs.

On the Ukrainian side, Ms Bogner reported “credible allegations” of summary executions of Russian prisoners among other abuses.

-with AAP