The International Court of Justice at The Hague has begun hearing a complaint filed by the Democratic Republic of Congo against Uganda.
Uganda is accused of massacring Congolese civilians
DR Congo accuses its neighbour of invading its territory, committing human rights violations and massacring Congolese civilians.
It is also demanding reparations for destruction and looting allegedly carried out by Ugandan troops.
Uganda denies the claims and accuses the DR Congo of acts of aggression.
In opening remarks, DR Congo's ambassador to the Netherlands, Jacques Masangu-a-Mwanza, told the court that Uganda was supplying weapons to ethnic groups fighting in Ituri Province.
"Uganda is behind a network of warlords who Uganda continues to supply with arms. They continue to steal our natural resources," he said. In 1999 the DR Congo asked the court to put a stop to acts of aggression by Uganda, which it said were a serious threat to peace and security in central Africa.
In a provisional ruling in 2000, the Court ordered both sides to refrain from any conflict - which could aggravate the case.
Last year, the DR Congo, Uganda and Rwanda started peace negotiations.
The DR Congo filed a similar complaint against Rwanda with the World Court in 2002.