An offensive by rebels has approached a major eastern DR Congo city and threatens to choke exports of tin ore, gold and other minerals. Fighters loyal to Congolese warlord Bosco Ntaganda have drawn within 40 kilometres of Goma, the capital of the country’s North Kivu province and a transit point for mineral exports, according to military and aid officials. United Nations peacekeepers, backing Congolese troops, have deployed tanks and helicopter gunships in an effort to protect the city of around 800,000 people from the rebel advance.

“Rebels are controlling most roads north of Goma,” Emmanuel Ndimubanzi, the head of North Kivu Miners Association told Dow Jones. “They are not safe for traders and miners.” In recent days the M23 rebel group has quickly captured several towns including Bunagana and Rutshuru.

The conflict between government troops and the rebels has forced up to 200,000 refugees to flee the fighting since April, seeking refuge in Uganda and Rwanda, according to the United Nations refugee agency, the UNHCR.

As a result of global “conflict minerals” concerns, the DRC’s share in the world tin market has dropped from a peak of 4% in 2008 to around 1.5% in 2011. Meanwhile the proportion of DRC cassiterite exported via Goma has dropped from 70% to a little over 30%, according to official export figures, as activity at the Bisie artisanal mining site declined sharply and the country’s tin business came to be focussed in the relatively safer Katanga province. Official trade data shows that a little over 1,000 tonnes of cassiterite (approximately 600-650 tonnes contained tin) was shipped from Goma in January-April this year.