At least 18 people have killed in clashes between the Democratic Republic of the Congo's army and rebels loyal to a renegade former general in the country's eastern region, according to military sources.

The latest round of violence comes less than three weeks before crucial elections to replace long-time president Joseph Kabila.

Fighting killed 14 rebels and four soldiers in Fizi, a region of South Kivu, a mineral-rich province which is prone to ethnic tensions, a military spokesman and other sources said.

Violence in the troubled eastern region is just one complication before December 23 elections in DR Congo, which has not seen a peaceful transfer of power since independence from colonial Belgium in 1960.

Under international pressure, Kabila has agreed to step aside, but critics worry he will try to engineer a win for his handpicked successor, former hardline interior minister Emmanuel Ramazani Shadary.

The eastern clashes pit the military against rebels loyal to a former army general, William Amuri Yakutumba, whose militia is one of the armed groups operating in the region against Kabila.

The militia is allied to rebels of the National Liberation Front based in neighbouring Burundi, according to several regional sources.

In September last year, the Yakutumba rebels attacked the town of Uvira on Lake Tanganyika near the Burundian capital Bujumbura.

United Nations forces pushed them back with Congolese support.

In February, the military said they had wiped out the Yakutumba group with help from Burundi, where some of them had taken refuge.

Western powers are closely watching the presidential election in the nation, where a UN peace-keeping force, now with 16 000 troops on the ground, has been operating for two decades.

Kabila's second and final elected term finished nearly two years ago, but he has remained in office thanks to a caretaker clause in the constitution.