The head of Nato has said the blast that fell in Poland killing two people was likely caused by a Ukrainian air defence missile - but that Russia was still ultimately responsible.
The explosion in the village of Przewodow has highlighted the dangers of Russia's war becoming a wider conflict - either by a deliberate act of escalation by the Kremlin or an accidental one.
Nato Secretary General Jens Stoltenberg said on Wednesday: "Let me be clear this is not Ukraine's fault." He added that there was no indication Russia was preparing offensive military actions against Nato allies.
How has Nato responded?
Poland had initially suggested it would invoke Nato's article 4, which provides for consultations among allies in the face of a security threat, but as since signalled it will step back from that.
According to Stoltenberg, more alliance members have said they are ready to provide air defence means. He announced no immediate NATO measures but said a contact group on Ukraine, would meet later in the day, with the main focus on air defence.
A spokesperson for Germany's defence ministry said Berlin would offer support to the Polish air defence, while Lithuania said Nato should deploy more air defences on the alliance's eastern flank.
Is Russia to blame for the explosion in Poland?
Ukrainian president Volodymyr Zelensky initially decried the missile strike as “a very significant escalation” of his country’s war with Russia.
However, the initial investigation found that the missile that hit Poland was probably fired by Ukrainian forces at an incoming Russian missile.
Despite this, Western leaders have been clear that whoever fired the missile, Russia and President Putin would ultimately be held responsible for an incident arising from its invasion.
"Russia bears ultimate responsibility as it continues its illegal war against Ukraine," Stoltenberg said on Wednesday.
A spokesperson for Rishi Sunak also said: "Whatever the outcome of that investigation, Putin's invasion of Ukraine is squarely to blame for the ongoing violence."
Watch: Biden says it is 'unlikely' a missile strike in Poland was fired from Russia
What is Nato?
The North Atlantic Treaty Organization is a political and military alliance of 30 countries.
Nato was set up in 1949 to protect members against the Soviet Union, with 12 nations initially signing up to the North Atlantic Treaty in Washington DC.
These countries were the US, Canada, the UK, Belgium, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway and Portugal.
Nato's stated political objectives are to "promote democratic values", "enable members to consult and cooperate on defence and security-related issues", and to "prevent conflict".
The organisation says it "is committed to the peaceful resolution of disputes", but if diplomacy fails it will use its military power "to undertake crisis-management operations".
Nato has four multinational battalion-size battlegroups, or some 4,000 soldiers, led by Canada, Germany, the UK and the US in Latvia, Lithuania, Estonia and Poland.
Following Russia’s full-scale invasion of Ukraine in February 2022, Nato reinforced the existing battlegroups and agreed to establish four more multinational battlegroups in Bulgaria, Hungary, Romania and Slovakia
The troops serve as a "tripwire" for Nato's response force to come in quickly and bring more US troops and weapons from across the Atlantic.
What happens if a Nato country is attacked?
The collective defence clause of Nato's founding treaty – Article 5 of the Washington Treaty – is a provision that means an attack against one member is considered an attack against all of them.
This is a fundamental part of Nato and why it says it is a defensive alliance.
Nato says military operations are carried out under Article 5 or a United Nations mandate, alone or in co-operation with other countries and international organisations.
Last month, in direct response to the conflict in Ukraine, Nato said: "Allies are committed to deploying robust and combat-ready forces on the Alliance’s eastern flank. The eight battlegroups demonstrate the strength of the transatlantic bond and the Alliance’s solidarity, determination and ability to respond to any aggression.
"Many activities undertaken by Allies nationally also contribute to increased Allied activity in the eastern part of the Alliance. In response to Russia’s full-scale invasion of Ukraine, Allies have sent additional ships, planes and troops to NATO territory in eastern Europe, further reinforcing the Alliance’s deterrence and defence posture."
Crucially, Ukraine is not a member of Nato and therefore the alliance is not treaty-bound to defend it.
US president Joe Biden has said he will not send American or allied troops to fight Russia in Ukraine.
However, Kyiv is a close partner and was promised eventual membership of the alliance.
The 30-member Nato works with Ukraine to modernise its armed forces.
Sweden has applied to join Nato along with Finland – which shares a border with Russia – with Nato hoping the expansion will boost its eastern Europe defences.
However, alliance members Hungary and Turkey are currently blocking the accession.
Which countries are in Nato, and what date did they join?
1949: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the UK, the US.
1952: Greece, Turkey
1999: Czech Republic, Hungary, Poland
2004: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, Slovenia
2009: Albania, Croatia
2020: North Macedonia
Why does Putin see Nato as a threat?
Since the Cold War ended, Nato has expanded eastwards by taking in 14 new countries, including the states of the former Warsaw Pact and the three Baltic nations that were once in the Soviet Union.
Russia sees this as a threatening encroachment towards its borders and continues to say it was a betrayal of Western promises at the start of the 1990s – something Nato denies.
Ukraine is not a Nato member but has a promise dating from 2008 that it will eventually get to join.
Since toppling a pro-Russian president in 2014, Ukraine has become closer politically to the West, staged joint military exercises with Nato and taken delivery of weapons.
Kyiv and Washington saw these as legitimate moves to bolster Ukraine's defence after Russia seized the Crimea region in 2014 and provided backing to separatists in eastern Ukraine.
Putin believes Ukraine's growing ties with the alliance could make it a launchpad for Nato missiles targeted at Russia.
He said Russia needs to lay down "red lines" to prevent that.
Last month, Putin said Nato troops on the ground fighting the Russian army would be "a very dangerous step that could lead to a global catastrophe".
In a recent address, Putin also said: "In Nato documents, our country is officially and directly declared the main threat to North Atlantic security. And Ukraine will serve as a forward springboard for the strike."
But his demands that Ukraine drop its long-term goal of joining the Atlantic military alliance have been repeatedly rebuffed by Kyiv and Nato states.