A second American health worker has been infected with the deadly Ebola virus at a West African hospital, according to an aid group.

Nancy Writebol tested positive for the tropical disease at the same medical facility in Liberia where American doctor Kent Brantly contracted the virus, said Ken Isaacs, vice president of program and government relations with US-based Christian relief group Samaritan's Purse.

"It's been a shock to everyone on our team to have two of our players get pounded with the disease," Isaacs said. "We are hopeful and prayful."

Writebol was working as a hygienist helping to ensure that those entering and leaving the hospital's Ebola unit were decontaminated.

Isaacs confirmed that both Writebol and Brantly, the 33-year-old medical director at the Liberia centre, are being held in isolation and under intensive treatment.

Writebol's husband, David, confirmed that his wife was very sick and that he could not enter the same room as her because of the virus.

Early treatment leads to a higher chance of survival and Dr Brantly began to receive treatment straight away as he recognised his own symptoms.

"We are hopeful, but he is certainly not out of the woods yet," Melissa Strickland, spokesman for Samaritan's Purse, said.

More than 670 people have been killed in Liberia, Guinea and Sierra Leone in what has been declared the worst outbreak of the disease in history by the World Health Organisation (WHO).

The highly contagious disease is one of the deadliest in the world and does not yet have a known cure.

The symptoms of the virus include fever, sore throat, diarrhea and internal and external bleeding, with a 90 percent fatality rate.

The risks for health workers attempting to combat the disease are incredibly high. One of Liberia's top Ebola doctors died last week after catching the virus while caring for patients, according to a government official.