The Ebola outbreak in Congo has not yet stabilised and is still expanding, with transmission fuelled by population movement, a World Health Official said on Tuesday.

The Democratic Republic of Congo has confirmed 1 561 cases, including 506 deaths, in the worst-ever outbreak of the rare Bundibugyo species of Ebola for which there is no proven treatment or cure. 

"It is still in the expansion phase unfortunately. We would like to say it is stabilising but frankly we cannot say it yet," Dr Anne Ancia, WHO representative in the country, told reporters by video link from Bunia, at the epicentre of the epidemic. 

She said that major challenges remained, such as the near-saturation of some Ebola treatment centres with occupancy levels around 90%.

Another difficulty is that workers falling ill in the mining town of Mongbwalu are not seeking treatment locally, but instead travelling and spreading the disease to new regions, she said.

"Population movements, persistent insecurity and the fragility of the health system continue to complicate efforts to bring the outbreak under control," she said.