An attack on a Polish village on the frontier with Ukraine killed two people late on Tuesday, sparking fears of a major escalation between the Nato group of countries and Russia.
Polish President Andrzej Duda said there were "many indications that it was an air defence missile that unfortunately fell on Polish territory," and that it was not thought to be an intentional Russian attack.
On Wednesday, President Biden used stronger language, telling Nato partners that the fatalities were caused by a stray Ukrainian air defence missile but that Russia was to blame, due to the intensity of its missile barrage on Tuesday.
Russia has launched missile attacks from occupied areas of Ukraine since it invaded in February. It has also fired them from its neighbouring ally Belarus, and from aircraft and ships in the Black Sea.
It is not the first time since Russia invaded Ukraine that objects have entered Nato airspace. In March, a six-tonne unmanned reconnaissance drone streaked across Eastern Europe and crashed in the Croatian capital Zagreb.
Nato nations have so far supported Ukraine — a non-member state — with weapons and financial aid, but have drawn the line at sending it the longest range missile systems and advanced fighter jets and refused to support Ukraine’s calls to set up an air defence zone over its airspace.
An article 5 invocation from Nato member Poland could lead to a deeper involvement of the organisation in the conflict. At the present time Poland has only discussed the possibility of a meeting to discuss Article 4 of Nato's founding treaty, which is less severe than Article 5 and requires a meeting to discuss a "threat to the security of any of the [Nato] parties."
Here's what you need to know:
What is Nato's Article 5?
Poland is a member of Nato. In theory, this means a deliberate attack could trigger Article 5 of Nato’s 1949 founding treaty.
That treaty, according to Nato, represented “a pact of mutual assistance to counter the risk that the Soviet Union would seek to extend its control of Eastern Europe to other parts of the continent”.
Since then, Nato has provided a framework for “collective security,” a concept the organisation says is “at the very heart of Nato’s founding treaty”.
What is Nato's Article 4?
Article 4 of the Nato charter states that "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."
It was invoked by no less than nine East European Nato members including Poland at the onset of the Ukraine invasion. Article 4 meetings have in some cases led to military deployments, including one that resulted in Nato sending air defences to Turkey as the Syrian war escalated in 2012.
Article 4 was also invoked by Poland in 2014 when Russian forces invaded and annexed Crimea. The Polish government said at the time that the purpose of the meeting was to show a unified Nato response to Russia's actions.
When has Article 5 been used?
For many years, following the break-up of the Soviet Union in 1991, critics of Nato have argued that it no longer has a clear reason for existence.
That changed in 2001 after the September 11 terrorist attacks in New York, when the organisation informed the UN that it would invoke Article 5.
Since then, Nato has committed joint forces to Afghanistan and, to a much lesser extent Iraq, in the form of a small training mission to help the Iraqi Army.
But the Ukraine crisis has brought Nato’s original form back to the fore — as a conventional military alliance intended to deter foreign state aggression, rather than terrorism.
This role came into focus in 2014 when Russia annexed the Crimean Peninsula in Ukraine and was accused of supplying separatists in eastern Ukraine with arms.
Russia said that it needed to take security action in Ukraine to push back against Nato expansion — Ukraine is not a member of Nato but began talks about joining in 2008, when it applied to start the Nato Membership Action Plan.
Critics of Russia’s position say that after the break-up of the Soviet Union, Moscow signed a treaty with Europe allowing former member states to join international organisations.
Either way, Nato is once again positioning itself as a powerful military alliance, warning that any attack on a member will provoke the “collective security” response.
“If there is a military attack on Nato territory it would cause the invocation of Article 5, and we would bring the full force of the Nato alliance to bear in responding to it,” US National Security Adviser Jake Sullivan told CBS News.
