Reports that Russian missiles have crossed into Nato member Poland, killing two people, have prompted alarm over the possibility of the war in Ukraine escalating into a much wider conflict.

A violent act on a Nato member – which Ukraine is not, but neighbouring Poland is – could trigger Article 5 of the military alliance’s charter. This “one-for-all and all-for-one” principle of collective defence means an attack on one is seen as an attack on all, and all members are expected to respond to assist their ally.

What makes this especially worrying is triggering the article would increase tensions between the US and Russia, the world’s two biggest nuclear powers.

But, with details of the incident yet to be confirmed, the prospects of a World War Three-like scenario very much depends on whether it was a deliberate act or an error.

The military alliance goes by the acronym of Nato.

The North Atlantic Treaty Organisation was formed in 1949 to prevent a resurgence of nationalism and militarism in Europe after two world wars, and to deter the Soviet Union’s expansion.

Its membership has swelled to 30 member nations, and over the 1990s and 2000s its enlargement stretched further east to include the former Soviet republics of Estonia, Latvia and Lithuania. Poland joined in 1999.

Since the start of the Ukraine war, Sweden and Finland, which shares a 1,300-km (810-mile) border with Russia, have sought membership, marking a radical break in the policy of both north east European countries given their staunch military neutrality.

Members agree to mutual defence – military action – in response to an enemy attack. The principle goes: “An attack against one ally is considered as an attack against all allies.” This is Article 5 of the Nato constitution.

Because Ukraine is not a member of Nato, Western support for Volodymyr Zelenskyy has fallen short of putting their troops on the ground. US president Joe Biden has made clear his concern about further escalating tensions with Russian president Vladimir Putin.

Ukraine’s admission to Nato has been a stated goal going back to its constitution in 2002. And while Zelenskyy has announced Ukraine is formally applying for fast-track membership, it seems unlikely Nato will accept his application while a war is raging.

Nato invoked Article 5 for the first and only time in its history after the 9/11 terrorist attacks against the US.

Since the war began eight months ago, there have been fears clashes with neighbouring countries such as Poland would up the ante dramatically.

On Tuesday, two people were killed in an explosion in Przewodow, a village in eastern Poland near the border with Ukraine. The Associated Press cited a senior US intelligence official as saying the blast was due to Russian missiles crossing into Poland.

The Polish Foreign Ministry late on Tuesday said that a Russian-made missile fell in eastern Poland and killed two people.

The ministry said foreign minister Zbigniew Rau summoned the Russian ambassador and “demanded immediate detailed explanations”.

Biden, in his call with Polish president Andrzej Duda, “offered full US support for and assistance with Poland’s investigation” and “reaffirmed the United States’ ironclad commitment to Nato”.

Biden also spoke to Nato secretary general Jens Stoltenberg, who was among those to hold a call with Duda.

The Russian defence ministry denied being behind “any strikes on targets near the Ukrainian-Polish border” and said in a statement that photos of purported damage “have nothing to do” with Russian weapons.

Experts seem to think the gravity of the situation depends on whether it was a deliberate attack – either to escalate the war or to test the West’s response – or an accidental misfire. In any case, it is up to Poland to decide the next steps.

Poland could instead invoke Article 4 – consultation with fellow Nato members if they fear their territorial integrity has been breached. This would likely give Poland more clout within the alliance and encourage the West to offer more military support to both Poland and Ukraine – support that falls short of attacking Russia.

Fabrice Pothier, former director of policy planning for Nato, told Sky News: “It’s too early to say if this was an intended attack, an intended strike on a Nato territory, or whether it was the misfiring of a missile or a rocket.

“However, even if that were the case, I think there is enough grounds for triggering what is just below Article 5, which is Article 4, which is Poland asking all Nato members to come to the Nato headquarters to consult each other to assess the threat and to take concrete action.

“This can be defensive, not going after Russia, but augmenting Poland’s air defence and obviously also augmenting Ukraine’s air defence, because Ukraine is the de facto first line of defence of the alliance.”

In October, British defence secretary Ben Wallace explained that the government is “deliberately ambiguous” about scenarios under Article 5 being triggered.

He said: “Article 5 is very clear, an attack on one is an attack on all, that is solid and in concrete and will be stood by.

“Article 5 doesn’t necessarily mean all-out war, people sometimes think it means the whole thing, but nevertheless there would be a response to an attack on Nato.”