By Joe Bavier

RUTSHURU, Congo, Aug 31 (Reuters) - Congolese troop reinforcements hurried on Friday to head off renegade Tutsi army units after a day of fierce fighting, raising the prospect of widening conflict in volatile North Kivu province.

Soldiers and ammunition arrived by air to reinforce a military base at Katale where forces loyal to renegade Tutsi General Laurent Nkunda fought heavy battles against government troops on Thursday that sent civilians fleeing for safety.

Two more brigades, each of around 2,000 men, headed to the area from positions further north and in the city of Kisangani.

The clashes have raised fears of a return to ethnic conflict in volatile eastern Democratic Republic of Congo, which bore the brunt of a 1998-2003 war in which some 4 million people perished mainly through hunger and disease.

"We're ruling nothing out -- we are ready for anything," Colonel Philemon Yav, commander of Charlie Brigade which is based in the Katale camp, told Reuters by telephone from the town, around 60 km (38 miles) from the provincial capital Goma.

"We do not fear any attack -- it is they who will be afraid of us."

Nkunda and around 4,000 Tutsi troops deserted the Congolese armed forces (FARDC) in 2004 but were brought back into the army in special mixed brigades under a January deal brokered by neighbouring Rwanda's Tutsi-led government.

Nkunda, who claims he can draw on some 8,000 fighters, has accused President Laurent Kabila's government of supporting his enemies in the largely-Hutu Democratic Forces for the Liberation of Rwanda (FDLR) rebel group which operates in eastern Congo.

Tensions have risen in the past few months and witnesses said around 1,000 of Nkunda's fighters attacked Charlie Brigade's headquarters in Katale, before dawn on Thursday, triggering a day of heavy fighting.

"We know that there were heavy losses on the attacking side," Sylvie Van Den Wildenberg, North Kivu spokeswoman for the U.N. peacekeeping mission MONUC.

"The situation is now calm. Some people have started to come back. The FARDC have been reinforced with our help, but we are still closely monitoring the situation," she said.

DISPLACED

Stray bullets hit civilians in the nearby town of Masisi during several hours of intense machine-gun and heavy weapons fire, prompting nearly all its 10,000-odd inhabitants to flee.

Witnesses said some residents began to drift back into Masisi on Friday, but the plight of the displaced added to an already precarious humanitarian situation in eastern Congo.

The U.N. World Food Programme (WFP), which says 200,000 people have been displaced by violence in North Kivu this year, said it hoped to get supplies to Masisi.

"The roads are impassable due to extremely bad conditions and the fact that that is where the fighting has been. The only way to bring it in is by air, so we're looking at different possibilities," said Aya Shneerson, WFP programme director for North and South Kivu provinces.

U.N. helicopters were shuttling troop reinforcements, weapons and ammunition to Masisi from Goma and returning soldiers killed and wounded in Thursday's fighting.

Military sources said 1,500 reinforcements had arrived in Goma and had been told they would be flown to Masisi.

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