Liberia's chief medical officer has placed herself in quarantine for 21 days after her office assistant died of Ebola.

Bernice Dahn, a deputy health minister, told the Associated Press that she did not have any Ebola symptoms but wanted to ensure she was not infected.

The World Health Organisation says 21 days is the maximum incubation period for Ebola, which has killed more than 3,000 people across West Africa and is hitting Liberia especially hard.

Ms Dahn has represented Liberia at regional conferences intended to combat the epidemic.

She said she had also instructed her office staff to remain at home for three weeks.

The unprecedented scale of West Africa's outbreak has made it difficult to trace the contacts of victims and quarantine those who might be at risk.