Volunteers prepare to remove the bodies of people who died of Ebola in the village of Pendebu, Sierra Leone.

US health authorities have turned down a request from Nigeria for an experimental drug to combat Ebola, as there are not enough doses available.

The health minister of Nigeria had asked the US Centers for Disease Control and Prevention (CDC) for access to ZMapp, a serum made by San Diego-based company Mapp Biopharmaceutical Inc.

However a CDC spokesman was forced to turn down the request, saying that "there are virtually no doses available."

Dr. Anthony Fauci, one of the world's leading immunologists and the director of the US National Institutes of Health, added that, according to the manufacturer, it would take two to three months to produce even "a modest amount" of the drug.

ZMapp was administered to two American health workers who were brought back to the US after contracting Ebola in Africa.

The two doctors now seem to be recovering, but it is not known if the improvements are a direct result of the drug.

"We don't even know if it works," Fauci said.

ZMapp was still in its experimental phase when it was administered to Nancy Writebol and Doctor Kent Brantly. It has not yet been assessed for safety on humans or approved for use by the US Food and Drugs Administration.

President Barack Obama said: "I don't think all the information is in on whether this drug is helpful. What we do know is that the ebola virus - both currently and in the past - is controllable if you have strong public health infrastructure in place.

"Let's get all the health workers that we need on the ground. Let's help to bolster the systems that they already have in place.

"And then during the course of that process, I think it's entirely appropriate for us to see if there are additional drugs or medical treatments that can improve the survivability of what is a very deadly and obviously brutal disease."

Ebola is one of the deadliest diseases in the world and does not yet have a known cure. The symptoms of the virus include fever, sore throat, diarrhoea and internal and external bleeding, with a fatality rate approaching 90% (around 60% in the current outbreak).

Described by the World Health Organisation (WHO) as the worst in history, the current outbreak has killed almost 1,000 people since March.