An outbreak of the Ebola virus has killed 14 people in western Uganda this month, health officials say, ending weeks of speculation about an unknown illness that had many people fleeing their homes.

Ugandan officials and a World Health Organisation representative told a news conference in Kampala on Saturday that there was "an outbreak of Ebola" in Uganda.

"Laboratory investigations done at the Uganda Virus Research Institute have confirmed that the strange disease reported in Kibaale district is indeed Ebola hemorrhagic fever," the Ugandan government and WHO said in joint statement.

Officials urged Ugandans to be calm, saying a national emergency taskforce has been set up to contain the disease.

Kibaale is a district in midwestern Uganda, where people in recent weeks have been troubled by a mysterious illness that seemed to have come from nowhere. Health officials spent weeks conducting laboratory tests that were at first inconclusive.

Joaquim Saweka, WHO's representative in Uganda, said the origin of the outbreak had not yet been confirmed.

"A team of experts from the government, WHO and CDC (US Centers for Disease Control) are in the field and following up on all suspected cases and those who got into contact with patients," he said.

Officials told reporters in Kampala, the capital, that the 14 dead, including a family of 12, were among 20 reported with Ebola, which causes haemorrhagic fever and internal bleeding.

A health worker who treated patients infected by the disease was also among the dead.

There is no treatment and no vaccine against Ebola, which is transmitted by close personal contact and, depending on the strain, kills up to 90 per cent of those who contract the virus.

Kibale is near the Democratic Republic of Congo (DRC) where the virus emerged in 1976, taking its name from the Ebola River.

Ebola first broke out in Uganda in 2000, killing 224 people out of more than 400 infected in the northern district of Gulu.

The disease re-emerged in 2008 in a western district close to the border with the DRC, killing 40 people.