By Joe Bavier

KINSHASA, Oct 9 (Reuters) - United Nations aid workers struggled to deliver food to thousands of civilians cut off by fighting in Congo's North Kivu province on Tuesday as rebel soldiers battled government troops after abandoning a ceasefire.

The renewed clashes between fighters loyal to renegade general Laurent Nkunda and the Congolese army threatened to worsen an already dire humanitarian situation in the eastern border province of Democratic Republic of Congo.

Government forces fought Nkunda's Tutsi soldiers for a second day near the villages of Shasha and Kirotshe, some 30 km (19 miles) west of the provincial capital Goma, according to officials from the United Nations Mission in Congo (MONUC).

Aid workers also reported combat near the town of Mweso, around 80 km (50 miles) northwest of Goma.

The clashes occurred a day after Nkunda announced his rebels were abandoning a month-old ceasefire, heralding a resurgence of conflict in North Kivu, long a tinderbox of ethnic tensions.

U.N. World Food Programme (WFP) officials travelling with a convoy carrying 100 tonnes of food aid into North Kivu's Masisi district reported hearing machine gun and artillery fire and seeing an exodus of fleeing civilians.

"They called me from the road and said they could really hear the battle. They saw hundreds of families fleeing towards Goma," Aya Shneerson, WFP director in North Kivu, told Reuters.

The convoy was the first in more than a month to travel to Masisi, which has seen multiple skirmishes between government soldiers, Nkunda's fighters, local militia and Rwandan rebels.

Shneerson said the fresh fighting could threaten attempts to get through to around 150,000 people the WFP estimates are out of reach of humanitarian assistance.

"This is a serious blow. The fighting restarted just as we were trying to relaunch distributions outside Goma. We'll have to reevaluate now based on what is happening," she said.

DETERIORATING SITUATION

Nkunda, who says he led a 2004 rebellion to protect Congo's Tutsi minority in the east, accuses President Joseph Kabila's government and armed forces of supporting Rwandan Hutu rebels -- traditional ethnic enemies of the Tutsi.

Kabila denies supporting the Rwandan Hutu rebels, who are accused of involvement in Rwanda's 1994 genocide that saw the slaughter of 800,000 Tutsis and moderate Hutus.

The newly displaced civilians arriving at camps near Goma over the weekend joined 370,000 who had already fled their homes due to violence since the start of the year in North Kivu.

The province, which borders Rwanda and Uganda, was at the centre of Congo's 1998-2003 war, which killed some 4 million people, mostly from hunger and disease.

"If this deteriorates into new all-out conflict, we'll have tens of thousands of new displaced," Jens Hesemann of the U.N. refugee agency UNHCR said. "It will be very difficult to maintain minimum humanitarian standards," he added.

North Kivu experienced two weeks of heavy fighting in August and early September after Nkunda's men deserted national army mixed brigades they had joined as part of a January peace deal.

The U.N. brokered a limited Sept. 6 ceasefire between the government and the rebel soldiers that lasted nearly a month.

Abandoning the truce on Monday, Nkunda accused the army of attacking his positions.

The government has given an Oct. 15 ultimatum for Nkunda's troops to integrate into the national army or face tough action.

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