The charity Save the Children said on Monday it would leave no stone unturned to find out how a British nurse contracted Ebola at a facility in Sierra Leone.

Save the Children's Sierra Leone director Rob MacGillivray told BBC TV that the charity would investigate the circumstances surrounding the infection of Pauline Cafferkey who had worked for the charity at a treatment centre in the country.

"We have put in an extraordinary review to ensure that we do everything, leave no stone unturned, to be able to as far as possible identify the source of this infection," MacGillivray told the BBC.

Cafferkey, 39, is in a critical condition at the Royal Free Hospital in London, having been diagnosed with the disease last week after she returned to Britain from the West African country.

The West African Ebola outbreak was first identified in Guinea's remote southeast in early 2014. Guinea, Sierra Leone and Liberia have borne the brunt of the 20,000 infections and nearly 8,000 dead.

MacGillivray said the investigation would focus on how protective equipment was used and person-to-person contact.

The Royal Free, Britain's main centre for Ebola cases, successfully treated British aid worker William Pooley with the experimental drug ZMapp after he was flown back to Britain in August.

Cafferkey is being treated with blood plasma from an Ebola survivor and an experimental anti-viral drug.