The World Court has begun hearings to decide if it has jurisdiction in a case against Nato for the 1999 bombing of Yugoslavia during the Kosovo crisis.
The bombing campaign lasted from March to June 1999
Serbia and Montenegro, Yugoslavia's successor state, accuse eight Nato countries of genocide and violating international law on the use of force.
The Nato members, including the UK, France and Germany, say the court is not competent to hear the case.
The court is hearing from all sides this week before making a judgement.
The original complaint was filed by the former Yugoslavia just a month after Nato launched air strikes against the country in March 1999.
The air strikes were not authorised by the UN. However, the Nato members concerned said at the time that their actions were justified as they aimed to protect ethnic Albanians in Kosovo from attacks by Serb forces.
But lawyers for Serbia and Montenegro accuse the Nato members of deliberately targeting civilians and violating international obligations banning the use of force against another state.
The Nato countries are accused of deliberately inflicting "conditions of life calculated to cause the physical destruction of a national group".
"In bombing the Federal Republic of Yugoslavia, military and civilian targets were attacked. Great numbers of people were killed, including a great many civilians," Serbia and Montenegro argued in court documents.
The New York-based Human Rights Watch estimates that from March to June 1999, around 500 civilians were killed during the Nato strikes in Serbia and Kosovo.
Belgium was the first Nato country to state its objections before the World Court, officially known as the International Court of Justice (ICJ) in the Hague.
Belgium's lawyer argued that "there is no longer a dispute between the countries" and invited the court to "discontinue the case on all grounds".
The ICJ is also hearing from representatives of the other countries accused: Canada, France, Germany, Italy, the Netherlands, Portugal and the UK.
Initial charges against Spain and the United States have been dropped.
The ICJ is the United Nations' highest legal body to resolve disputes between nations.