The Democratic Republic of Congo accused Uganda of "barbarity" Tuesday in a fresh bid at the UN's top court to extract billions of dollars in compensation over a brutal war two decades ago.

The International Court of Justice ruled in 2005 that Uganda had to pay Kinshasa reparations for invading its vast central African neighbour during a 1998-2003 war that left three million people dead.

The case is now back before the Hague-based ICJ, which must make a final ruling on the amount of compensation after the two squabbling countries failed to reach an agreement through negotiations.

"The armed conflict led by Uganda was very large scale. A five-year occupation followed by very serious breaches of human rights that were verging on barbarity," the DRC's legal agent Paul-Crispin Kakhozi Bin-Bulongo told the court.

Congolese officials said at the time of the original decision that they would claim between $6 billion to $10 billion from Kampala.

They had expected Uganda to "fully assume its responsiblity for the injury caused and a substantial contribution", but that had not happened in the end, Bin-Bulongo added.

"The judgment said parties should seek agreement in good faith... Unfortunately Uganda never acted in the same spirit."

Kinshasa filed a new application in 2015, asking the 75-year-old world court to reopen the case and to make a finding on reparations as negotiations failed.

The ICJ, in its 2005 ruling, however also said that Uganda was entitled to compensation after its embassy in Kinshasa was attacked and its diplomats abused.

The conflict at it its height drew in some nine African countries, with Uganda and Rwanda backing rebel forces against the Kinshasa government as they jostled for control of the mineral-rich Ituri region.

Uganda's representatives will state their case for a counter-claim on Thursday.

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