"Marburg is a very bad haemorrhagic fever, even worse than Ebola," said Allarangar Yokouibd from the UN World Health Organisation (WHO) on Friday.
"We have had several Ebola epidemics in the region but none with such a high mortality rate," he said at a news conference in Luanda.
In Geneva, the WHO said the world's worst outbreak of the virus was not under control.
Mike Ryan, head of the WHO's emergency response unit, said international agencies and local health authorities must remain firmly engaged in Angola for the next four to six weeks, adding that the situation in Angola was not yet under control.
The world body on Friday launched an emergency appeal for $3.5 million to intensify the fight against the outbreak - recorded as the largest ever recorded and still growing.
"The appeal for $3.5 million will enable UN agencies, including the WHO, Unicef and the WFP to support the Angolan government intensifying outbreak control efforts," said the UN's resident co-ordinator in Angola, Pierre-Francois Pirlot.
"It is clear that this epidemic is unprecedented not only in Angola, but everywhere. It is the biggest epidemic of haemorrhagic fever so far"
UN's co-ordinator in Angola
"It is clear that this epidemic is unprecedented not only in Angola, but everywhere. It is the biggest epidemic of haemorrhagic fever so far," he said.
Most of the victims come from the northern town of Uige, the centre of the outbreak about 300km north of the seaboard capital, Luanda. About 200 cases had been reported since the outbreak began in October.
"The victims included nine health workers, seven nurses and two doctors," said Angolan Vice Health Minister Jose Van Dunem.
Fear has gripped the capital and the country of 14 million people, which emerged three years ago from a brutal 27-year civil war.
The Marburg virus, whose exact origin is unknown, spreads through contact with bodily fluids such as blood, excrement, vomit and saliva, and can be contained with relatively simple health precautions, according to experts.
Yokouibd said the disease was transmitted through contact with all bodily fluids, including the sweat and tears of a sick person. He stressed it was not an airborne disease.
He said the worst outbreak of Ebola, in the same family as Marburg, was in Uganda, where 404 cases were reported at the end of the outbreak.