The president of South Sudan is directly profiting from the country’s civil war and even his 12-year-old son has a stake in a business venture, according to a two-year investigation commissioned by the actor and activist George Clooney.

Salva Kiir, his former deputy Riek Machar and associates of both men have looted the country in accumulating wealth that includes multimillion-dollar mansions, top-of-the-range cars and stakes in a number of overseas businesses, a report by the US-based watchdog The Sentry claims.

Some of the family members and close associates have posted photos of themselves on social media on planes, in five-star hotels and in luxury vehicles even as South Sudan descends into a “violent kleptocracy” that has claimed tens of thousands of lives and forced 2.5 million people from their homes.

South Sudan gained independence from Sudan in 2011, when Hillary Clinton was secretary of state, but plunged into conflict soon after Kiir fired Machar as vice-president in 2013. Both sides are accused of orchestrating mass rapes, child soldier recruitment and massacres of civilians. A peace deal reached a year ago under international pressure has been violated repeatedly by fighting, and Machar fled the country last month.

“The simple fact is they’re stealing the money to fund their militias to attack and kill one another,” Clooney told a press conference in Washington yesterday before a meeting with Barack Obama.

“The evidence is thorough, it is detailed and it is irrefutable. It involves arms dealers, international lawyers, international banks, international real estate and it is because of these international actors that we are also able to provide solutions to help end this criminal behaviour to protect innocent civilians,” he said.

George Clooney: ‘The evidence is thorough, it is detailed and it is irrefutable.’ Photograph: Molly Riley/AFP/Getty Images

The report, entitled War Crimes Shouldn’t Pay, says Kiir earns about $60,000 per year but alleges he has spent a fortune on properties outside the country including a two-storey, 460-square-metre villa in the gated community of Lavington, a comfortable area of Nairobi. Machar reportedly has a home in the same neighbourhood.

The document says: “The key catalyst of South Sudan’s civil war has been competition for the grand prize, control over state assets and the country’s abundant natural resources between rival kleptocratic networks led by President Kiir and (former) Vice-President Machar.

“The leaders of South Sudan’s warring parties manipulate and exploit ethnic divisions in order to drum up support for a conflict that serves the interests only of the top leaders of these two kleptocratic networks and, ultimately, the international facilitators whose services the networks utilize and on which they rely.”

Kiir’s 12-year-old son held a 25% stake in a holding company formed in February this year. Overall, there was evidence that at least seven of Kiir’s children, as well as his wife, have held stakes in various business ventures, the report said. Journalists were shown several passports for individuals who gave their occupation as “president’s son”.

Clooney’s fellow actor and campaigner Don Cheadle said: “The Sentry has found evidence that these top officials responsible for mass atrocities in South Sudan have managed to accumulate fortunes and have been involved in illegal transactions, insider deals and outright fraud.

“Immediate family members of President Kiir and his wife have held interests in almost two dozen companies operating in oil, mining, construction, gambling, banking, telecommunications, aviation and government and military procurement.”

The Sentry said its undercover investigators pored over thousands of pages of legal records, corporate filings, financial statements and other official correspondence, tracked suspects on social media and used satellite imagery to gather and analyse data. The researchers travelled to locations including Melbourne, Adelaide, Kampala, Juba, Cairo and Nairobi to gather evidence and interview hundreds of experts and eyewitnesses.

The investigation focused on top officials with command authority over those responsible for mass atrocities, he added. They have accumulated personal fortunes despite modest salaries. Gen Paul Malong, the chief of military staff, owns two villas in Uganda in addition to a $2m mansion in a gated community in Nairobi, according to the report, which cites his annual salary as roughly $45,000.

Among Kiir’s associates, Lawrence Lual Malong Jr, 28, describes himself as “Young Tycoon” and “Smart Boy for Life”, according to the report, which includes social media photos of him wearing gaudy blue, purple and yellow suits in the first class section of various planes.

Veteran activist John Prendergast said: “This is not a study of a few corrupt officials in Africa … What it is is an attempt to get at the nexus of what drives violent conflict and mass atrocities in South Sudan: the connection between endemic corruption and deadly violence and about how these networks benefit from a system that’s built on corruption and uses extreme violence to keep in place.”

The report claimed individuals and major firms outside South Sudan had facilitated the deadly corruption. It said there was hard evidence of foreign companies making direct payments to the bank accounts of high-ranking South Sudanese generals. The banks that process the transactions also play a role, it said. Cheadle added: “These companies and banks can no longer say they didn’t know.”

Clooney and his colleagues said they would present the findings to Obama and urge the international community, including South Sudan’s neighbours, to crack down on banks that fail to stop dubious transactions, and impose asset freezes on those responsible for human rights violations.

Contacted by the Guardian, Ateny Wek Ateny, a spokesperson for Kiir, said he was reading the report. “We will be able to respond tomorrow. Not now.”

