Health care workers in Liberia have administered three doses of the rare, experimental drug ZMapp to three doctors suffering from Ebola, two medical workers in Monrovia told agencies.

Liberia, the West African country with the highest death toll from the tropical virus at 413, received three doses of the rare serum in a special consignment this week.

Doctors Zukunis Ireland and Abraham Borbor from Liberia and Dr Aroh Cosmos Izchukwu from Nigeria are the first Africans to receive the treatment. The drug has already been administered to two American healthcare workers and a Spanish priest, all previously working in Liberian hospitals.

The US healthcare workers’ health has since improved but the Spanish priest died.

"Three doctors are currently being administered treatment with the experimental Ebola drug ZMapp. Treatment began on Thursday evening,” said Dr. Billy Johnson, chief medical officer of John F. Kennedy Medical Centre in Monrovia where two of the doctors served before contracting the deadly virus.

A second healthcare worker at the Elwa centre which is housing the sick doctors confirmed that they were on their third day of a six-day ZMapp treatment.Details of their condition are not known.

The UN health agency said only around 10 to 12 doses of the drug have been made and this raises difficult ethical questions about who should get priority access.

The apparent improvement in the two US healthcare workers’ condition has stoked popular pressure to make the drug available to Africans - a cause advocated by the Twitter hashtag group #giveustheserum.

There is currently no vaccine against the highly-contagious disease and other forms of treatment are only designed to relieve symptoms such as fever, vomiting and haemorrhaging.

Up to 90 per cent of victims die – a fatality rate so high that the U.S. Centers for Disease Control and Prevention (CDC) classifies the illness as a category A "bioterrorism agent” – although the current outbreak fatality rate is near 60 per cent.