Angola is one of the world's biggest oil producers

A statement from the office of the president dismissed reports by lobby group Human Rights Watch as an attempt to tarnish its image.

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.

Malnourished

"The government can't be held responsible for estimated income that is based on non-credible sources, bearing in mind that none of the international financial institutions have to date proven those accusations," the statement said.

The statement added that no independent audit has ever been done to prove the accusations levelled against the government.

Most of the population remains poor

The Human rights Watch 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.

"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.

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.

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.