Nato has convened an emergency summit after a “Russian-made” missile possibly fired by Ukraine crashed in Poland killing two in a “serious escalation” of the war in Ukraine.

The world nervously awaited last night after US intelligence initially reported the weapon had been fired by the Russians – an act that could have been seen as an attack on the West. It was the first time a missile has strayed over the border from Ukraine since the invasion in February.

US officials however have now suggested the missile was fired by Ukrainian forces at an incoming Russian one amid the salvo against power infrastructure Tuesday. Nato member Poland however has put its military on high alert – with a deliberate, hostile attack on the country potentially triggering a collective military response by Nato.

Russia and the West remain at loggerheads over the war in Ukraine – with nations such as the US and UK offering weapons and support to Kyiv. Moscow has always been furious about this – and so the spectre of Putin’s invasion of Ukraine spilling into a wider war, or even World War Three, has cast a long shadow over Europe.

In a ratcheting up of tensions, one of Putin’s close and senior allies Dmitry Medvedev warned the world is heading towards World War Three. The deputy chair of Russia’s top security council seemed to blame the strike on the West.

“The incident with the Ukrainian-alleged ‘missile strike’ on a Polish farm proves just one thing: waging a hybrid war against Russia, the West moves closer to the world war,” wrote Medvedev.

But in a major de-escalation in the early hours of this morning, US President Joe Biden said that it was “unlikely” that the missile was fired from Russia.

Biden called an emergency meeting of the G7 and Nato leaders – and the military alliance will meet again in Brussels today at 9am. Nato sources reportedly said the US president told allies the blast had been caused by a Ukrainian air defence missile. Ukraine has vowed to assist with the investigation.

Downing Street however said that whoever fired the missile, Putin is “squarely to blame for the ongoing violence”. Prime Minister Rishi Sunak stressed that the incident took place against a barrage of civilian infrastructure in Ukraine – and said the investigations continue.

“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.”

The confusion likely stemmed from the fact that Ukraine still maintains stocks of former Soviet and Russian-made weapons – including the S-300 air-defence missile system.

Poland is continuing to investigate the attack after the rocket struck a farm at 3.40pm local time on Tuesday in the village of Przewodów, four miles from the border with Ukraine.

Pictures from the scene showed a huge crater, an overturned trailer and a damaged tractor on the farmland were the missile struck. The victims of the explosion were two men, a farmer and an employee of the local grain plant – both died at the scene.

“I heard a big bang, it hit about a kilometre from my house, I was scared that it was the war,” a local told Polish online news Onet.

The moment of the explosion was also described by a 28-year-old resident of Przewodów: “I was on the balcony, I didn’t see it flying, but I heard an explosion: first one, then the other. It was still quite clear,” he said.

“Most of the residents evacuated. When my wife came back from shopping, we also decided to leave. We took the most necessary things and got into the car. The policeman asked us where we were going, but when he saw the children, he let us through.”

• The Sun report