The Rwanda-backed M23 armed group has taken control of the mining hub of Walikale in DR Congo, local sources said Thursday, despite attempts to broker a ceasefire this week.
The seizure of the town of around 60,000 people late on Wednesday marks the farthest west the anti-government group has advanced into the interior of the Democratic Republic of Congo since it emerged in 2012.
It comes just after Congolese President Felix Tshisekedi and his Rwandan counterpart Paul Kagame held surprise talks in Doha on Tuesday, expressing their support for a ceasefire.
But the terms of any truce remain unclear, with mediator Qatar saying further negotiations were necessary.
"Walikale-centre is occupied by the M23. We retreated to avoid human losses," an officer in the DRC's military (FARDC) told AFP, saying its forces were now around 30 kilometres away in Mubi.
A separate security source confirmed the capture and also said fighting took place in Mubi on Thursday.
The offensive had already caused mining group, Alphamin, this month to evacuate its employees and halt operations at the world's third most productive tin mine.
The Bisie site produces the tin ore cassiterite and is located in the Walikale district of North Kivu province.
The halt in mining drove up prices of tin, while concerns rise over the supply chain of the valuable metal used to solder electronic components onto printed circuit boards.
The boom in the electronics and renewable energy sectors is fuelling growing demand, according to analysts.
The region also has several gold mines.
The M23 fighters "are in the neighbourhoods of Walikale", Fiston Misona, a civil society representative from the community, said early Thursday.
Another resident, who spoke on condition of anonymity, said they saw groups of armed fighters "through the windows" of their house.
A Doctors Without Borders (MSF) base was "caught in the crossfire" during the fighting but no injuries were reported, local official Marco Doneda said.
"The MSF team is concerned about the influx of those wounded in the coming days and hours," he said.
The M23 has waged a lightning push over the last few months in the mineral-rich eastern DRC, driving the Congolese army out of much of North and South Kivu provinces and raising fears of a wider regional war.
The DRC government has accused Rwanda of backing the M23 in order to seize valuable mineral resources.
Rwanda denies providing the M23 with military support, but a UN experts' report has said that Rwanda maintains around 4,000 troops in the DRC's east to assist the armed group.
On Tuesday, Kagame and Tshisekedi met in Doha for talks mediated by Qatar's Emir Sheikh Tamim bin Hamad Al-Thani.
The two African heads of state whose previous attempts at talks collapsed at the last minute -- expressed their support for "an immediate and unconditional ceasefire", a statement from the three countries said.
No details have emerged on the terms of how the ceasefire will be implemented, as it would also have to involve the M23 on the ground to succeed.
The meeting between the two heads of state came after peace talks between Kinshasa and the M23, due to have been held in the Angolan capital Luanda on Tuesday, were cancelled.
The last time the Congolese government and the M23 held talks was in 2013.