A US doctor in Liberia infected with the deadly ebola virus has insisted that the only available dose of an experimental serum be given to a fellow American aid worker afflicted with the disease.

Dr. Kent Brantly and missionary Nancy Writebol were part of a team from two North Carolina-based Christian relief groups, Samaritan's Purse and SIM who were responding to an outbreak of the virus.

Franklin Graham, president of Samaritan's Purse, said in a statement: "Yesterday, an experimental serum arrived in the country, but there was only enough for one person. Dr Brantly asked that it be given to Nancy Writebol. Even as he battles to survive ebola, this heroic doctor is still focused on the wellbeing of others."

Doctors have reported a slight improvement in 33-year-old Dr Brantly's condition, however he remains serious.

Writebol, who helped disinfect the protective suits worn by medical personnel such as Brantly inside the isolation ward at a care centre in Monrovia, Liberia, is said to be "fighting through" the disease.

Ken Issacs, vice-president of Samaritans Purse said: "We think it was in the scrub-down area where the disease was passed to both Nancy and Kent."

Two other American volunteers remain in isolation in West Africa amid fears they too could have contracted the virus, while one US citizen has died from ebola, in Nigeria, after taking a flight to the region.

The latest reports of victims of the virus have emerged as the World Health Organisation (WHO) announced a $100m plan to tackle the outbreak.

The Centres for Disease Control and Prevention have recommended against non-essential travel to Guinea, Liberia and Sierra Leone.

Within the region, measures have bee taken to contain the spread of the disease. Sierra Leone has declared a state of emergency and called in troops to quarantine ebola victims. Neighbouring Liberia has closed schools and instructed public servants to stay home from work in an effort to halt the spread of the disease. .The Sierra Leone football squad has, been prohibited from travelling to the Seychelles for an African Cup of Nations qualifier. The Peace Corps said it was evacuating 340 volunteers from Liberia, Guinea and Sierra Leone.

Responding to the crisis, WHO Director-General Margaret Chan will meet in Conakry, Guinea, with the presidents of the affected West African nations, in a bid to address the growing threat.

"The scale of the Ebola outbreak, and the persistent threat it poses, requires WHO and Guinea, Liberia and Sierra Leone to take the response to a new level and this will require increased resources, in-country medical expertise, regional preparedness and coordination," she said.

There is currently no vaccine for the virus which is spread via direct contact with bodily fluids such as urine, blood, sweat or saliva. 60% of cases are fatal.

The ebola outbreak is the worst in history and has already killed 729 people in West Africa, according to WHO.

As health officials warn that the disease could spread worldwide, hospitals in the UK are also taking precautions against any ebola outbreak.