By John Chiahemen

KINSHASA, May 12 (Reuter) - The deadly Ebola virus, one of the most lethal diseases known to man, has spread to a third town in Zaire and a leading official expressed concern that families of victims were dumping them in hospital and fleeing.

But the World Health Organisation and the aid group Medecins Sans Frontieres (MSF), who have foreign experts battling the disease on the ground, remained confident they would be able to contain the outbreak which has killed at least 29 people.

Two medical experts from the U.S. Centres for Disease Control and Prevention in Atlanta and one from the Institut Pasteur in Paris rushed from Kinshasa to Kikwit, where the virus surfaced in March.

"We will stay for as long as it is necessary," one of the three told Reuters. He declined to be named.

"The situation is so urgent," said a U.S. embassy spokesman in Kinshasa, adding that foreign embassies were pledging aid.

The virus, named after a river in north Zaire, normally hits monkeys and other animals but sometimes jumps to humans. Outbreaks in Zaire and Sudan in 1976 and 1979 were contained by public health measures, but still killed hundreds of people.

With Kikwit under quarantine and the authorities in the teeming capital blocking roads from the affected region, the WHO said in Geneva on Friday that the virus had spread.

It listed four hospitals where it had been identified. Two in Kikwit, a town of 500,000 people some 500 km (310 miles) from Kinshasa, one at Mosango 100 km (60 miles) away and one at Yassa Bonga, 250 km (160 miles) from Kikwit.

"The pattern is of infectious patients being transferred to these hospitals from the 350-bed Kikwit General Hospital where most of the initial cases appear to have occurred," a WHO statement said. Health workers are particularly at risk.

The virus, which causes a form of haemorrhagic fever and for which there is no known cure or vaccine, kills as many as nine out of 10 people who contract it. "Transmission of the virus through close contact is continuing to occur," the WHO said.

Kinshasa governor Bernadin Mungul Diaka said the authorities were concerned that families were dumping sick relatives in hospital and fleeing. "Those who have died from the disease were abandoned by their families," he told Reuters.

The WHO and MSF say public health measures can contain the virus, which spreads through close contact with blood or bodily fluids. "I think it can be confined and there will not be a large outbreak all over the place," Dr Eric Verschueren, the head of the MSF medical mission to Zaire, said in Brussels.

The WHO's Geneva statement, based on a report from its team in Zaire, said 27 people had died from Ebola and 22 others were hospitalised, "many of them in terminal stages of illness". MSF put the death toll at 29. Three Italian nuns are among the dead.

Kinshasa's governor has closed roads to the city from Bandundu province, which produces almost half of its food.

"I have barred all movement of people into Kinshasa from Bandundu (province)," Mungul said on Thursday, adding that he had stopped small aircraft flying in from the zone.

"If the disease penetrates to Kinshasa that will be a catastrophe," he said, adding that the mortuary in the city of five million people had room for only 150 corpses.

The government declared Kikwit a disaster zone and slapped a quarantine order on the town. "The movement of people either entering or leaving is subject to sanitary control,"it said.

Zaire's old colonial ruler Belgium has taken steps to ensure passengers arriving by plane from Zaire do not bring the virus.

Officials had reported at least 90 deaths in Kikwit but blamed some on an associated outbreak of bloody diarrhoea there.