The United Nations says the Democratic Republic of Congo's army has clashed with rebels in some of the worst fighting in a week despite the rebel leader's promise to support a cease-fire.

The two sides battled on Sunday night in Rwindi, about 75 miles north of the eastern provincial capital of Goma, causing about 150 people to take refuge outside a UN peacekeeping base and there was no immediate word on casualties.

Meanwhile, the World food Programme has started a five day operation to distribute food to 12-thousand people in Rutshuru and Kiwanja, scene of some of the fiercest fighting in the latest upsurge in the war in eastern DRC.

WFP spokesperson Marcu Prior said the organisation had been able to move truck loads of food across the frontline north of Goma and into the two places for the first time.

"This is vitally needed because before the fighting there was 60-thousand, at least, people living displaced here and dependent on WFP for their food needs, so we needed to get back here as quickly as we could."

By the end of the operation, on Wednesday, it was hoped that some 93-thousand displaced and host families would have received a 15-day ration.