The death toll from the ebola epidemic rose to 4922 out of 10,141 known cases in eight countries, the World Health Organisation (WHO) said last night.

The virus, which reached Mali through a two-year-old girl who died on Friday, now threatens Ivory Coast, having infected people virtually all along its borders with Guinea and Liberia. The three worst-hit countries of West Africa - Guinea, Liberia and Sierra Leone - account for the bulk of the world's worst ebola outbreak, recording 4912 deaths out of 10,114 cases there, the WHO said in its update.

The group's overall figures include outbreaks in Nigeria and Senegal, deemed by the WHO to be now over, as well as isolated cases in Spain, the United States and a single case in Mali.

However, the true toll may be three times as much, while the death rate is thought to be about 70% of all cases.

The WHO has said that many families are keeping infected people at home rather than putting them into isolation in treatment centres - some of which have refused patients due to a lack of beds and basic supplies. The United Nations agency, sounding an ominous note, said that out of the eight districts of Liberia and Guinea sharing a border with Ivory Coast, only two have yet to report confirmed or probable ebola cases.

It has also said trials of ebola vaccines could begin in West Africa in December, a month earlier than previously expected, and hundreds of thousands of doses should be available for use by the middle of next year.

The WHO says 15 African states including Ivory Coast are at the highest risk of the deadly virus being imported. In the last 10 days it has sent teams to Mali and Ivory Coast to help national authorities gear up their capacity to detect and treat potential cases. Four WHO experts are travelling this weekend to Mali to reinforce the team there.

The agency warned that many people in Mali had potentially been exposed to the virus because the little girl was taken across the country while she was ill. Some 43 people with whom she was in contact, including 10 healthcare workers, are being monitored.

In all, 450 healthcare workers have been infected to date - including one in Spain and three in the US - leading to the deaths of 244 of them, the WHO said.

The governors of the US states of New York and New Jersey have ordered a mandatory 21-day quarantine period for all doctors and other travellers who have had contact with ebola victims in West Africa.

Anyone arriving from affected West African countries without having had confirmed contact with ebola victims will be subject to monitoring by public health officials.

The move follows the diagnosis in New York of Dr Craig Spencer, who had been working in Guinea.

The first person to be quarantined under the rules was a female health worker who arrived at Newark Liberty International Airport on Friday.

She had no symptoms then, but later developed a fever. A preliminary test came back negative for ebola, the New Jersey health department said yesterday, but the woman remains in isolation.

Also in the US, two nurses infected while caring for dying Dallas patient Thomas Eric Duncan have been declared free of the virus.