The US government is stepping up production of ZMapp, believed to be one the most promising of the Ebola drugs currently under development.

The Department of Health and Human Services is said to be in "advanced" discussions" with Texas-based drugmaker Caliber Biotherapeutics to produce the drug in tobacco plants.

According to the newspaper New York Times, two of the world's biggest charities, the Bill & Melinda Gates Foundation and Wellcome Trust are also looking to have Zmapp produced in animal cells, the more conventional method.

A cocktail of three different monoclonal antibodies, Zmapp is being developed by San Diego-based Mapp Biopharmaceutical, with financial support from the US and Canadian governments.

Some medical experts believe that the drug is the best shot at Ebola as the antibodies attach themselves to the virus and neutralize it.

At present, only limited quantities of the drug, which US officials said has shown success in monkeys and may also have contributed to the recovery of two American aid workers who contracted the disease in Africa, are available.

The Zmapp doses used to treat the Americans were produced by Caliber - which also manufactures proteins, including antibodies in tobacco leaves at a plant in Owensboro, Kentucky, owned by Reynolds America. However, the facility can only turn out 10-20 treatments a month.

US health officials have said they will also look at other experimental production channels.

In the UK, GlaxoSmithKline is fast-tracking an experimental Ebola vaccine of its own, co-developed with the US National Institutes of Health (NIH).

Canada's Tekmira Pharmaceuticals also has developed Ebola treatments said to be advanced enough to be tested on people. In August, US authorities were reported to have expressed safety concerns about the Canadian drug, but it was unclear what these were.