The world was on a knife edge today as leaders from Nato countries discussed what to do after a missile of unconfirmed origin landed in a Polish border town, killing two people.

Initial reports from an unnamed senior US intelligence official and the Ukrainian government pointed the finger at Russia. But later US President Joe Biden said the blast may not have been caused by a missile from Russia.

Poland, a NATO member, has launched an investigation and is considering whether to invoke elements of Nato's collective defence treaty, which regards an attack on one of its members as an attack on all.

So what could happen next?

University of Waikato International Law Professor Al Gillespie talks to Lisa Owen.