JOHANNESBURG — A rights group on Monday called on Angola to drop criminal defamation charges against an investigative journalist who wrote a book on human rights abuses in Angola's diamond-rich region.

Rafael Marques de Morais attended a hearing on July 31 for 10 new lawsuits that were brought against him, along with one pre-existing suit, Human Rights Watch said. The lawsuits revolve around a book that alleges Angolan generals own a diamond company and a security firm that carried out killings and the torture of workers toiling in the southern African nation's mines.

Marques is one of Angola's most prominent investigative journalists, who has also exposed corruption cases and human rights violations through his blog, the group said. Several high-ranking Angolan generals brought the suit against Marques. The journalist had filed a criminal complaint against the plaintiffs in 2011, which were shelved.

The rights group called on Angola to repeal its criminal defamation laws, which are the basis for the charges against Marques. Neither Marques nor his lawyer were allowed to review the materials related to the lawsuits, Human Rights Watch said.

"Angola has found its criminal defamation laws very useful to try to squelch reports about corruption and human rights violations," said Human Rights Watch deputy Africa director Leslie Lefkow. "Angola should be investigating these reports of serious human rights violations instead of trying to silence the bearers of bad news."

A number of journalists have been prosecuted in recent years by senior government officials for defamation, which is a criminal offense in Angola, the group said.

Marques was sentenced to six months in prison and payment of damages after being convicted of defaming President Jose Eduardo Dos Santos in 2002. The U.N. Human Rights Committee ordered Angola in 2005 to pay damages to Marques for wrongful conviction, though the government has yet to carry out those orders, Human Rights Watch said.

In February, Portuguese prosecutors threw out a libel suit brought by nine Angolan generals against Marques. His book, "Blood Diamonds: Corruption and Torture in Angola," was published in Portugal, Angola's former colonial ruler.

The Lisbon Attorney General's office issued a ruling that said it decided the book falls within the scope of legitimate use of a legal right — freedom of expression and information — which is constitutionally guaranteed.

Angola's government is accused of corruption and mismanagement of the country's oil and diamond riches.

The country was a Cold War battlefield for 27 years, with Cuban soldiers and Soviet money supporting dos Santos' government and apartheid South Africa and the United States backing UNITA. Half a million people died in the war, and more than 4 million — a third of the population — were displaced and much infrastructure was destroyed.

Since the war ended in 2002, Angola has dominated the list of the world's fastest growing economies and is sub-Saharan Africa's second-largest oil producer, after Nigeria. Oil-backed credit lines from China — Angola is China's No. 1 oil supplier and its second biggest importer is the United States — have fueled a building boom of houses, hospitals, schools, roads and bridges.

But 87 percent of urban Angolans live in shanty towns, often with no access to clean water, according to UNICEF, and more than a third of Angolans live below the poverty line. Meanwhile, human rights activists accuse government and military officials of looting their country's oil and diamond wealth.