The president of Sierra Leone has written to Prime Minister Tony Abbott, making a desperate plea for military aid to combat the Ebola virus outbreak.

The president of Sierra Leone has made a desperate plea for Australia to scale up its response to the Ebola crisis, including sending military aid, as the deadly virus continues to ravage West Africa.

In a letter addressed to Prime Minister Tony Abbott, which arrived this week, President Ernest Bai Koroma says his country is counting on Australia and specifically requests military aid, warning Sierra Leone is losing the battle against Ebola.

The development came as Foreign Minister Julie Bishop on Thursday announced Australia would immediately boost its financial contribution to fighting the worst ever outbreak of the deadly disease by another $10 million, taking the total commitment to $18 million.

However, the Australian government has so far ruled out sending medical experts and logistical support.

The refusal by Australia to provide medical experts and logistical support has prompted criticism from aid organisations, including Save the Children and Medecins Sans Frontieres.

In the letter dated September 18, sent through diplomatic channels, Mr Koroma warns the nation’s health system had already been overwhelmed by the virus which, according to the World Health Organisation, has claimed 3338 lives and infected 7178 since the beginning of the year.

“While we are doing everything possible to stop the outbreak, further support is urgently needed from your friendly government to scale up our national response with … education efforts, as well as infection control measures,” the letter says.

Mr Koroma makes a specific request for Australia to deploy military health units, logisticians and engineers.

“Having watched the response of the Australian military to similar humanitarian emergencies, most recently Typhoon Haiyan in the Philippines, I know that it is uniquely placed to help us in the fight against Ebola.

Mr Koroma says in the letter that Australian military aid could potentially help save thousands of lives.

“We are counting on Australia to send us the military personnel we so desperately need to fight back against the virus and prevent the positive developments of the last 10 years from being undone.”

Ms Bishop on Thursday said the government has assessed that financial contributions were the best and most efficient way Australia could make a rapid contribution to the global response to the crisis.

But Save the Children and Medecins Sans Frontieres, while welcoming the additional aid money offered on Thursday, criticised the Australian government’s refusal to do more, as other world leaders deploy troops and medical experts in their thousands.

The US has committed up to 3000 troops while the UK will spend $185 million on its mission, including supporting 700 Ebola treatment beds across Sierra Leone.

“Make no mistake, this crisis is at tipping point. We need to act urgently and decisively,” Save the Children acting chief Mat Tinkler said.

The UN is seeking $US50 million ($A54 million) from donors to meet immediate needs over the next four weeks, including for logistics to deliver equipment, materials and supplies to Ebola response operations.