A missile that killed two people near Poland’s border with Ukraine threatened to be “a very significant escalation” in the war with Russia overnight as world leaders met to assess the situation.

Rishi Sunak said the UK would “support our allies” as Joe Biden pledged US support for Poland but stated it was “unlikely” Russia fired a missile.

The president’s assessment was later backed up preliminary assessments from US intelligence suggesting the missile was fired by Ukrainian forces at an incoming Russian one.

Joe Biden issues update on investigation into explosion in Poland

The Prime Minister joined an emergency morning meeting on the incident at the G20 summit in Bali.

He was pictured sitting next to the US President, who called the roundtable of likeminded G7 and Nato leaders.

Ukrainian President Volodymr Zelensky condemned the attack as “a very significant escalation”, but Russia denied any involvement in the Poland blast.

The Polish government said it was investigating and raising its level of military preparedness.

Poland and Nato used language that suggested they were not treating the missile blast as an intentional Russian attack, with Nato calling it a “tragic incident”.

If Russia had deliberately targeted Poland, it would risk drawing the 30-nation alliance into the conflict at a time when Moscow is already struggling to fend off Ukrainian forces.

Where is Poland on the map?

Polish media said the strike hit an area where grain was drying in Przewodow, a village near the border with Ukraine.

Poland is the closest Nato country to Ukraine, sharing an Eastern border with Ukraine, Belarus and Lithuania.

The Argus:

What is article 5 and how does it relate to the explosion in Poland?

Article 5 is part of the North Atlantic Treaty which in principle means an attack against one NATO member is considered an attack on all.

Discussions around Article 5 surfaced after initial news broke suggesting Russia was responsible for the explosion however latest reports suggest this is not the case.

Article 5 was not invoked when Russia invaded Ukraine due to the fact Ukraine is not an existing member, however Poland is so anything found to be an attack on Poland could lead to Article 5 being invoked although that seems unlikely at this stage.

What is Article 4 and does it mean the UK could go to war?

Article 4 is the start of formal consultations with the threatened member of NATO about the risks they face.

NATO members will “consult together” but that does not mean discussions will lead to conflict with all decisions being made by consensus.

NATO’s website reads: "The parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the parties is threatened."

Key questions remain around the strike which came amid a barrage of Russian air strikes across Ukraine.

Three US officials said preliminary assessments suggested the missile was fired by Ukrainian forces at an incoming Russian one amid the targeting of Ukraine’s electrical infrastructure on Tuesday.

That assessment and Mr Biden’s comments at the G20 summit in Indonesia contradict information earlier on Tuesday from a senior US intelligence official who told the Associated Press that Russian missiles had crossed into Poland.

In an official read-out of the meeting, issued by the European Union, the leaders said they agreed to offer their “full support for and assistance with Poland’s ongoing investigation”, while reaffirming their “steadfast support for Ukraine and the Ukrainian people in the face of ongoing Russian aggression”.

What has Rishi Sunak said about the explosion in Poland?

Rishi Sunak has accused Russia’s President Vladimir Putin of showing “utter contempt” for the international order as Western leaders vowed to establish how a missile came to hit a Polish village close to the Ukrainian border.

Speaking at the end of the G20 summit in Bali, the Prime Minister said there were already teams on the ground trying to work out what happened, amid reports it may have been a Ukrainian missile fired in defence against a Russia attack.

However, Mr Sunak stressed that the incident took place against a barrage of more than 80 Russian missiles launched against civilian infrastructure in Ukraine.

“That was happening at a time when the G20 was gathered trying to find resolution to some of the world’s challenges and the same time Putin was raining down indiscriminately that volume of missile attack,” he said.

“I think it shows utter contempt for the international rules-based system.”