Already 2,700 trucks carrying around 89,000 metric tons of copper are stuck in Kolwezi, two of the sources said.

The strike could tighten global supplies of copper, needed for power generation and construction, and cobalt, used in the rechargeable batteries that power electric vehicles. Both materials are crucial for the energy transition.

A Reuters survey this month showed expectations for a small copper market surplus of around 112,000 tons this year, which could become a deficit if the strike is prolonged.

DRC is the world's third largest copper producer, accounting for 10.4% of 22 million metric tons of global mined copper supply last year, according to the International Copper Study Group. The country is also the world's largest cobalt producer.

Truck drivers, who move minerals from Kolwezi to Zambia then to coastal ports for transporting by vessel to consuming countries, such as China, have demanded logistics firms pay an extra risk allowance of $700 per journey, the sources said.

The strike has stopped material produced by major suppliers, including Ivanhoe Mines' giant mine Kamoa Kakula, CMOC's Tenke Fungurume, Glencore's Kamoto and Sicomines' Mashamba West from leaving Congo.

Glencore declined to comment. CMOC, Ivanhoe and Sicomines did not respond to requests for comment.

"The truck drivers want an incentive for them to operate in Congo, they want a danger allowance on top of their current salary," Hippy Tjivikua, chief executive, at Walvis Bay Corridor Group said.

"It's affecting most of the supply routes. I can't say cargo is completely blocked, but most of the mines aren't able to go and offload or pick up the loads."

The danger allowance is payment for the risks associated with the trucks being robbed of high value copper and cobalt materials.

The central African country supplied the world with 76% or 141,500 tons of cobalt last year, mining and metals company Darton Commodities said.

(Reporting by Julian Luk; addition reporting by Felix Njini; writing by Pratima Desai; Editing by Veronica Brown and Barbara Lewis)

By Julian Luk and Felix Njini