A surge in new cases of Ebola in Liberia has seen the number of people believed to be infected pass 4,200, according to World Health Organisation figures.
The new toll includes more than 500 new cases in Liberia in just a week.
The UN health agency has said that it expects thousands of new infections in Liberia, the hardest hit country in the current outbreak, in the coming weeks.
New figures published by the WHO attribute more than 2,200 deaths to Ebola.
Earlier, the African Union said it had agreed to send medical support teams to the West African countries affected by Ebola for a six-month mission.
The Ebola outbreak began in Guinea and has since spread to Liberia, Sierra Leone, Nigeria and Senegal.
The hardest hit of those countries desperately need more healthcare and humanitarian workers to respond to the crisis.
The African Union is sending at least 100 people, including doctors and nurses, to support efforts to contain the outbreak.
The European Union promised €5m to fund the mission following an emergency meeting on Monday.
The teams will work in Liberia, Sierra Leone and Guinea.
Meanwhile, WHO staff battling the Ebola outbreak in Sierra Leone are working in larger quarters and no longer live with people from other agencies, after a scientist with the UN group was infected with the disease last month, a spokeswoman said.
A second WHO healthcare worker, a doctor, has now also been infected in Sierra Leone, the UN health agency announced on Monday.
After a Senegalese epidemiologist with the WHO tested positive for the disease, the agency conducted an investigation into how he became infected.
While the WHO is not releasing the results of the investigation, spokeswoman Nyka Alexander said that staff living and working quarters in Sierra Leone have been expanded to make them less cramped and they no longer share living space with officials from other agencies.
Changes were also made to working procedures, including more routine temperature checks for everyone coming to the WHO office and living quarters.
She said the investigation report was “pretty clear” about revealing how the Senegalese epidemiologist was infected but said the agency would not be releasing details.
“It’s not a new or unexpected risk,” she said. Epidemiologists do not treat patients but are sometimes involved in contact tracing to follow up potential cases and liaising with safe burial teams for Ebola victims.
Ms Alexander said a second investigation has now begun into how the latest WHO doctor was infected and the agency has also recommended the Sierra Leone government stop accepting new patients into the Ebola clinic where the incident occurred.