More than $4bn of oil revenue has disappeared from government accounts in Angola over a five-year period, says the lobby group Human Rights Watch.
Angola is one of the world's biggest oil producers
The shortfall - equivalent to 10% of Angola's annual income - came from analysis of International Monetary Fund figures, Human Rights Watch said.
The group said the money had been lost through corruption and mismanagement.
Angola is one of the world's biggest oil producers but after 27 years of war, most of the population are poor.
"While ordinary Angolans suffered through a profound humanitarian crisis, their government oversaw the suspicious disappearance of a truly colossal sum of money," said Arvind Ganesan, director of the Business and Human Rights program at Human Rights Watch.
The report said the money had gone missing from 1997 to 2002 and was equivalent to the amount the government spent on social programmes during that period.
Some 900,000 Angolans are internally displaced, while 3.7m children are malnourished, the United Nations says.
Human Rights Watch said any further aid to Angola from the international community should be conditional on strict requirements for transparency in the government budget.
Most of the population remains poor
Last year, the IMF sent a team to Angola to investigate reports that $900m of oil money had gone missing in 2001 alone.
At the time, the Angolan Government insisted the shortfall was due to accounting problems.