The World Health Organisation says the number of people believed to have Ebola has risen above 10,000.

The UN health agency said the number of confirmed, probable and suspected cases has risen to 10,141. Of those, 4,922 people have died.

The WHO has said repeatedly that even those high figures are probably an under-estimate as many people in the hardest-hit countries have been unable or too frightened to seek medical care.

The Ebola outbreak in west Africa is the largest ever seen. It has particularly affected Guinea, Liberia and Sierra Leone, but there have also been cases in three other west African countries, Spain and the US. Mali recorded its first case on Thursday.

A shortage of labs capable of handling potentially infected blood samples has made it difficult to track the outbreak. For example, the latest numbers show no change in Liberia's case toll, suggesting the numbers may be lagging behind reality.

On Thursday, authorities confirmed that the disease had spread to Mali, the sixth west African country affected, and on the same day a new case was confirmed in New York, in a doctor recently returned from Guinea.

Mali had long been considered highly vulnerable to the disease, since it shares a border with Guinea. The disease arrived there in a two-year-old, who travelled from Guinea with her grandmother by bus and died yesterday.

The toddler, who was bleeding from her nose during the journey, might have had high-risk contact with many people, the WHO warned. So far, 43 people are being monitored in isolation for signs of the disease, and the WHO said authorities are continuing to look for more people at risk.

To help fight Ebola, the UN humanitarian flight service airlifted about a ton of medical supplies to Mali late yesterday. The seats of the plane were removed to make room for the cargo, which included hazard suits for health workers, surgical gloves, face shields and buckets, according to the World Food Programme, which runs the flights.

"Speed is of the essence in this Ebola crisis. Agencies such as WFP and WHO are working every hour to confront together the virus as a matter of priority," said Denise Brown, the west Africa regional director for the UN food agency.