Some of the world's best known consumer electronics companies are examining their supply chains after discovering that tin from a mine controlled by renegade soldiers in the Democratic Republic of Congo is reaching international markets.

The Bisie mine, situated a day's walk into the Walikale district of North Kivu, is the Congo's biggest tin ore mine. It is controlled - in contravention of the country's mining regulations - by the renegade 85th brigade of the Congolese army led by Colonel Samy Matumo.

An investigation by the Financial Times has found that tin ore from the mine is reaching smelters and solder manufacturers in Asia via a network of local middlemen and international traders. Household brands such as Hitachi (NYSE: HIT - News) , Microsoft, Pioneer (NYSE: PIO - News) and Samsung are investigating whether they could be sourcing tin solder from these companies.

Congo's mines ministry last month suspended all activities in the Walikale district to "restore order," leading to a rise in world tin prices. Prices for tin ore, or cassiterite - used to make tin solder used in the manufacture of electronic components - reached a high at the London Metal Exchange of more than $18,000 per metric tonne.

The war in Congo between 1998 and 2003 was perpetuated by the fight between Congolese armed factions and armies from neighbouring countries for control of mineral wealth. In an ominously familiar repeat of this pattern, the 85th brigade has been funding itself through the sale and control of the cassiterite trade at Bisie.

A recent report commissioned by the British government's Department for International Development estimated that Bisie produces 10,600 tonnes of Congo's total exports of 16,870 tonnes. While this is a fraction of total world production of 260,000 tonnes, the revenues generated are significant.

During a recent visit to Bisie, the FT learnt that 85th brigade soldiers are confiscating cassiterite mined by artisanal miners, some of whom are as young as 12. The brigade then sells the cassiterite to other traders. It also generates revenue at illegal road blocks along mineral transport routes.

The experts that investigate diversions of natural resources in Congo for the United Nations stated in February that "individuals and entities buying mineral output from the eastern part of the DR Congo with a strong rebel presence are violating the sanctions regime when they do not exercise due ­diligence".

Enrico Carisch, a former member of the group of experts, commented that the "uncontrolled trade and lack of due diligence in the trading chains of cassiterite is directly linked to the disproportionate militarisation of North Kivu", where renegade soldiers threatened last year to upset international efforts to restore peace.

Cassiterite from Bisie is bought by middlemen linked to exporters and international traders who sell the ore on to smelters that purchase on the open market. At the smelters, the tin from North Kivu is mixed with other tin, refined and sold either directly to solder manufacturers, or through international metal exchanges. Finally, tin solder is sold to manufacturers for use in the production of electronic gadgets.

Alexis Makabuza Ruzenga, a purchaser and exporter of tin ore from Bisie and founding shareholder of the co-operative Comimpa that works at the mine, told the FT that he sells tin ore directly to the Malaysia Smelting Corporation Berhad, based in Malaysia.

In an e-mail to the FT, the company denied that it had traded with him. However, it confirmed that it sources approximately 12 per cent of its tin ore from Congo. Other smelters in Asia also purchase cassiterite from North Kivu. They then sell refined tin to solder manufacturers, including Japan-based Nihon Superior, which confirmed that it supplies companies including Hitachi, Kenwood, Panasonic, Pioneer and Sony.

In response to the FT's queries on its supply chain, Microsoft stated that "we don't have visibility into the activities of commodity suppliers participating at the beginning of the hardware supply chain".

Samsung said that it had now requested that its component providers investigate their suppliers of tin and stated that it is working closely with the Electronics Industry Code of Conduct Extractive Work Group to find the best solution to the problems of sourcing from countries such as Congo.

Hitachi stated that it "will review levels of compliance amongst its primary suppliers with these guidelines and ensure that business practice standards are met by all companies operating within the supply chain".

Pioneer stated that it "takes any alleged breach of the code of conduct seriously and will investigate further".