A missile that killed two people in Poland close to the border with Ukraine has sparked Nato crisis talks and an emergency meeting of world leaders at the G20 summit.

The explosion hit a grain facility amid a barrage of around 80 missiles launched by Russia at Ukrainian infrastructure targets in what is believed to be one of the largest salvos of the war so far.

While early reports from senior US intelligence officials suggested that Russian missiles had crossed into Poland, three US officials have since revealed that preliminary assessments suggested the missile was launched by Ukrainian forces to intercept incoming Russian rockets.

Following an urgent meeting of G7 and Nato leaders in Bali, including Prime Minister Rishi Sunak, US President Joe Biden said it is “unlikely” that the missile was fired from Russia.

Here is what we know so far of this developing story.

Where did the missile land?

Two people died in the explosion in the Polish village of Przewodów in the east of the country, about four miles from the Ukrainian border.

A resident said the two victims were men who were near the weighing area of a grain facility when they were killed on Tuesday afternoon.

Footage shows a huge bomb crater at the site of the blast, which was initially blamed on Russian forces.

Poland is a Nato member, with Article 4 of the alliance’s treaty calling on member countries to determine whether Nato’s territory is threatened.

Under Article 5 of the defensive alliance, an attack on one country is considered to be an attack on all member states. Yesterday marked the first time the war had spilled over into Nato territory.

The deaths came as Russian President Vladimir Putin launched a wave of missile attacks on Ukrainian civilian infrastructure targets in cities including Kyiv and Lviv, knocking out power supplies.

Russia blamed in initial assessments

Last night, a senior US intelligence official claimed that Russian missiles were to blame for the blasts in Poland.

Poland’s prime minister Mateusz Morawiecki called an urgent meeting of national security and defence officials and the Polish government also said it was “raising the readiness of military units” in response to the “crisis situation”.

A spokesman for the US State Department, Vedant Patel, described the incident as “incredibly concerning” and said Washington was “working to determine what happened and the appropriate next steps”.

Russia denied that it had carried out any strikes near the Ukrainian- Polish border and claimed the reports were “deliberate provocation”.

Ukrainian President Volodomyr Zelensky urged Nato allies to respond to the “significant escalation” after the deaths in Poland.

But Biden says it’s “unlikely” Russia to blame

The US President later said that initial findings suggested it was “unlikely” Kremlin forces had launched the strike on Poland.

Speaking after a meeting with G7 and Nato leaders to discuss the incident, Mr Biden said: “There is preliminary information that contests that.

“I don’t want to say that until we completely investigate it, but it is unlikely in the lines of the trajectory that it was fired from Russia, but we’ll see.”

According to the Associated Press, three US officials said preliminary assessments suggested the missile was launched by Ukrainian forces at an incoming Russian strike.

Although that has not yet been confirmed, it would change the response of Nato to the fatal blast if it was found to be an accident.

What has Rishi Sunak said?

Mr Sunak was woken at 5am to help deal with the crisis and held a call with the Foreign Secretary and Defence Secretary to discuss their response, before speaking to Poland’s President Andrzej Duda on the phone.

G7 and Nato leaders who met in Bali agreed that Mr Sunak and his Canadian counterpart, Justin Trudeau, should place calls to the foreign minister and president of Ukraine because of the UK and Canada’s role in supporting the country.

Speaking alongside Mr Trudeau in Bali on Wednesday, Mr Sunak said: “I think the right thing now is for everyone to just calmly ascertain exactly what happened and gather the facts.

“There are teams that are doing that; we’ll stay in touch, as we’re talking both to the Ukrainians, to the Poles and indeed allies amongst ourselves. Until we have a definitive answer, it is right everyone just remain calm.”

UK officials have refused to confirm reports from the US that the missiles appear to have been fired by Ukrainian forces in order to intercept Russian rockets, but it is understood the British Government has no reason to doubt the findings.

Western officials have been privately relieved by the increasing possibility that Russia was not directly responsible for attacking Poland, which would have further escalated the conflict.

The Prime Minister said that no matter who filed the missiles, Vladimir Putin bore ultimate responsibility. He told reporters: “I think the most important thing to recognise is the reason Ukraine is having to use missiles is to defend its homeland.

“It’s is having to defend its homeland against an illegal and barbaric set of strikes by Russia.”

At the start of Mr Sunak’s meeting with the US President, Mr Biden said: “It’s merciless. I mean it borders on — it’s way over the top. At the moment where world leaders meeting here in Bali are seeking progress on world peace, Putin’s striking civilian targets – children, and women.”

In a press conference this morning, Mr Sunak said “we should all be clear – none of this would be happening if it weren’t for Russia’s invasion of Ukraine”.

He added “it was important to establish the facts” around the missile that landed in Poland.

What are Nato doing?

An emergency meeting is taking place of all 30 Nato member states in Brussels this morning. The crisis talks will be chaired by Secretary-General Jens Stoltenberg.

Article 4 of the NATO charter says that member states “will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security” of another member is threatened.

The 30 member states then start formal consultations at the request of the threatened member. The talks look at whether a threat exists and how to counter it, with decisions arrived at unanimously. Article 4 does not, however, mean that there will be direct pressure to act.

Following the strike, Polish officials had said they were likely to invoke Article 4 but there were indications on Wednesday morning that Poland would not now ask for Article 4 to be deployed.

This consultation mechanism has been triggered several times in Nato’s history. For instance, by Turkey one year ago, when Turkish soldiers were killed in an attack from Syria. At that time, Nato decided to consult, but did not take any action.

If it was determined that Moscow was to blame for the blast in Poland, Article 5 could be triggered by Nato, starting deliberations on a potential military response.

The only time that Article 5 was used was in 2001 after the 9/11 attacks on the US. When the US then attacked Afghanistan, Nato sent a mission alongside.