United Nations peacekeepers in the Democratic Republic of Congo have made an "urgent appeal" for militias to stop fighting in the east.
Congo's new national army has incorporated many rival factions
The appeal was made as former Rwandan-backed rebels clashed with army troops for a third day near the town of Goma blocking their advance.
Tensions have risen in recent weeks after Rwanda threatened to send troops into DR Congo.
Three million people died in DR Congo's five-year war, which ended in 2002.
The fighting drew in at least six other armies, including that of Rwanda, which has twice invaded its much larger neighbour in recent years.
"[UN mission in DR Congo] Monuc launches an urgent appeal for calm to all the factions engaged in this fighting and for an immediate cessation of hostilities," the mission said in a statement issued in the capital, Kinshasa.
Reuters news agency quotes a senior official in North Kivu as saying the fighting had resumed on Monday afternoon.
"We still don't have an idea of casualties, but the fighting is heavy and heavy weapons are being used," said Albert Semana, the province's director of security.
The former RCD rebels are reported to be preventing the advance of the regular troops towards the Rwandan border.
The RCD troops were supposed to have been integrated into the Congolese army under a peace deal to end the war.
The BBC's Arnaud Zajtman in Kirotshe, in North Kivu, says hundreds of civilians have abandoned their villages.
One soldier was killed and two were wounded in the clashes, he says, and many homes in the area have been looted by the Congolese army.
Rival forces fought for about three hours in Kirotshe on Saturday morning.
People in Eastern Congo are being preyed on by rival armed groups
On Sunday, there were reports of troops clashing in Kanyabayonga, some 70 miles (113km) north of DR Congo's main eastern city, Goma.
Our correspondent says the fighting in Kirotshe was sparked by the presence of Rwandan soldiers in the area.
DR Congo's President Joseph Kabila has said that 10,000 army reinforcements will be deployed to the east after Rwanda repeatedly threatened to send troops over the border to hunt down Hutu rebels based there.
Rwanda has twice invaded DR Congo, ostensibly to hunt down Hutu extremists.
These rebels are accused by Rwanda of being the remnants of the army which committed genocide there in 1994 before the forces of the current Rwandan government chased them into DR Congo.