The world’s supply of tantalum, an essential component in most computers and mobile phones, is under threat as armed rebels in eastern Democratic Republic of Congo have encircled a key trading hub.

Since last month, the M23, a rebel group that Congo says is backed by neighboring Rwanda, has blocked trade routes to the city of Goma and is helping to smuggle tantalum from some of the country’s richest deposits, according to Congolese government and military officials and United Nations experts.

“They’re taking it to Rwanda,” likely through Virunga National Park, Lt. Col. Guillaume Ndjike Kaiko, a regional spokesperson for Congo’s military, said in an interview in Goma, the lakeside capital of North Kivu province that borders Rwanda. Rwanda’s government denies the accusations.

Tantalum is on a list of mineral resources that have raised international red flags for helping to fund years of conflict in Congo. For over a decade, industry groups that include companies like Intel Corp. and Apple Inc. have made efforts to ensure their mineral supply chains are conflict-free.

However, the renewed M23 offensive, focused on North Kivu, has left more than a million people in squalid camps and is tainting the region’s mineral output. Last week, the Responsible Minerals Initiative, which helps more than 400 of the world’s biggest corporations avoid purchasing metals that fuel or fund violence, warned its members about sourcing metal from the region in letters seen by Bloomberg.

The US and European Union have laws discouraging companies from purchasing minerals linked to the conflict. Virginia-based RMI did not respond to an email requesting comment.

Congo and Rwanda were the world’s top two sources of tantalum in 2023, according to US Geological Survey estimates.

Congolese government data show North Kivu accounted for about half the country’s supply to the world in 2022. The province’s exports dropped 59% last year as the M23 advanced and an ownership dispute erupted over the country’s largest mine, Rubaya, according to a provincial mining report seen by Bloomberg.

Decades of Conflict

The current fighting in North Kivu has its roots in the mid-90s when the aftermath of the genocide in Rwanda spread across the border. Rwanda and the M23 say Congo’s government still protects militias with links to that violence, which killed an estimated 800,000 Rwandan Tutsis and moderate Hutus. The M23 is mainly led by Tutsis from Congo.

More than 100 armed groups are now active in the region, clashing over land, economic and political access and ethnic grievances.

Most mining at Rubaya and nearby tantalum sites is done by hand and is currently overseen by a loose coalition of armed groups known as Wazalendo, allied with Congo’s army, according to a United Nations experts’ report. However, the output now ends up in areas controlled by the rebels they’re fighting, Congolese officials say.

The M23 had limited involvement in the mineral trade when it launched its most recent rebellion in late 2021. That seems to have changed since the beginning of the year as the group encircled Goma — North Kivu’s main trading hub.

Official tantalum shipments from the province almost entirely stopped after M23 moved south and took control of the town of Shasha last month, cutting off the last main transport route to Goma from the Rubaya area, according to Ndjike and provincial mining statistics.

For Congo, the costs of the spiraling violence are massive. As locals flee their towns and villages for safety in displacement camps, aid groups say responding to the crisis will cost billions of dollars.

“The objective here is minerals, it’s to get the minerals that are found in our zone of responsibility,” Ndjike said of the current wave of fighting. “And it has an impact that’s not only economic but also humanitarian.”