Good news from Congo. And perhaps even more unexpectedly, it has come about with the help of users of mobile phones, legislators in Washington and corporate giants such as Apple and Intel. That is the finding of an investigation by the Enough Project, an anti-genocide campaign group, which says Congolese warlords have lost their grip on most of the country's mines and lucrative conflict minerals.

The mines have long been held up as a case of corporate social irresponsibility. Adults and often children spend days underground digging up the tin, tantalum and tungsten used to make computers and mobile phones, creating billions in profits for electronics companies.

Moreover, the minerals also generated $185m (£110m) every year for armed groups responsible for atrocities including killings and rapes, the report says, fuelling one of the world's longest conflicts in eastern Democratic Republic of the Congo.

Yet in an apparent triumph for consumer activism, and blow to political cynicism, the militias have lost control of more than two-thirds of mines in the past four years, according to the Enough Project. Among the warlords to cede control is Bosco Ntaganda, nicknamed "The Terminator", who once profited handsomely from illegal mines but is now facing trial at the international criminal court.

The turning point was a US law introduced in 2010, that required companies to determine the origin of minerals used in products. Brought in under the Dodd-Frank Financial Reform Act, it required any company that might be using conflict minerals to register with the US Securities and Exchange Commission and publish its supply chain. The firms, presumably aware of the public relations implications of being linked to murderous warlords and child labour, have generally complied.

Now, for the first time in Congo's history, there is a validation process to evaluate mines as conflict-free, and 112 out of 155 mines surveyed passed as clean. Intel is producing the world's first fully conflict-free product that contains clean Congolese minerals, the report says, while Apple has validated its tantalum supply chain as conflict-free. "This is spurring other companies to accelerate their reform efforts."

Meanwhile, another shaft of light has appeared in the past year as UN peacekeepers, acting on a more aggressive mandate, took on and defeated two of the rebel groups that enjoyed access to mines. According to the Enough Project, communities near conflict-free mining projects now experience a greatly reduced presence of armed groups, while hospitals and schools are starting to be built in those areas. The wages of miners have also risen, in some cases threefold, and an increased number of miners now receive helmets and safety equipment and experience safer working conditions.

"Our research found that electronics companies are expanding their responsible minerals sourcing from Congo, and Congolese miners are now able to earn 40% more from those mines," said Sasha Lezhnev, senior policy analyst at the Enough Project. "Mines formerly controlled by warlords such as Bosco Ntaganda are now part of peaceful supply chains, as 21 electronics brands and other companies now source from 16 conflict-free mines in Congo."

The warlord Bosco Ntaganda, pictured in 2009, used to derive great profits from mines in the DRC that he would plough into his armed group. He is currently on trial at the ICC in The Hague. Photograph: Lionel Healing/AFP/Getty Images

Speaking from Goma, field researcher Fidel Bafilemba, a co-author of the report, expressed confidence that similar models can succeed elsewhere. He said the initiative was built on "similar and earlier campaigns against cheap labour in the east Asian textile industry, and others. Pressuring companies to humanise their supply chain works."

The US-based Enough Project said it conducted five months of field research in eastern Congo, interviewing 220 people in 14 mines and towns, in addition to 32 interviews in the US and Europe. It found that 67% of tin, tantalum (refined from coltan) and tungsten in North Kivu, South Kivu and Maniema provinces were no longer in the hands of the armed groups or the Congolese army.

But inevitably there is a caveat. Artisanal mining of gold is still funding military commanders, the research found. Bafilemba added: "US special envoy Russ Feingold, the United Nations, and especially now those in jewellery business, must squarely address conflict gold that still funds armed groups responsible for atrocities and grave human rights abuses."

Another organisation that has long campaigned on the issue, Global Witness, agreed that the American legislation had a major impact but there is no room for complacency. "The US conflict minerals law has catalysed reforms in DR Congo's mining sector and has compelled US-listed firms to examine their sourcing practises for the first time, in a bid to ensure they are behaving responsibly and not funding war," campaigner Sophia Pickles said.

"But despite positive steps, there are still real challenges to overcome in DR Congo, not least the illegal involvement of high-ranking members of the Congolese army in the minerals trade.

"Some mining sites have been demilitarised, but our research shows that members of the Congolese army are profiting from the minerals trade in less visible ways, such as via taxation and extortion. The Congolese government must hold to account those acting illegally. Prosecutions in Congolese military courts are critical to cleaning up the minerals trade but to date the political will to make them happen is lacking."