The leader of Rwandan-backed M23 rebels in eastern Congo said on Thursday that a call by Kinshasa and Kigali for an immediate ceasefire "doesn't concern us" as his forces pushed deeper into Congolese territory by capturing the strategic town of Walikale.

Walikale is the farthest west the rebels have reached in a swift advance since January that has already overrun eastern Congo's two largest cities. The town of 15,000 people fell after fighting on Wednesday between the rebels and the army and allied militias.

The conflict, rooted in the fallout from Rwanda's 1994 genocide and competition for mineral riches, is eastern Congo's worst since a 1998-2003 war that drew in multiple neighbouring countries and resulted in millions of deaths.

With troops from Congo, Rwanda and Burundi having all participated in fighting this year, a conflict that has simmered for years is evolving into a wider regional war, experts say.

"The enemy now controls Walikale," Nestor Mavudisa, a spokesperson for Democratic Republic of Congo's army, told Reuters.

Walikale is in an area rich in minerals including tin and lies along a road that links four eastern Congo provinces.

Its capture puts the rebels within 400 km (250 miles) of Kisangani, which is the country's fourth-biggest city and has a bustling port at the Congo River's farthest navigable point upstream of the capital Kinshasa.

Congolese President Felix Tshisekedi and his Rwandan counterpart Paul Kagame called on Tuesday for an immediate ceasefire after a surprise meeting in Qatar's capital Doha, their first direct talks this year.

The leader of the M23 alliance dismissed the appeal, and said his forces were not fighting at Rwanda's behest.

"We are Congolese who are fighting for a cause," Corneille Nangaa, head of the Congo River Alliance (AFC), told Reuters in an interview in eastern Congo's biggest city, Goma.

"What happened in Doha, as long as we don't know the details, and as long as it doesn't solve our problems, we'll say it doesn't concern us."