The current Ebola outbreak is the largest on record. It has spread from Guinea to Sierra Leone, Liberia, Nigeria and Senegal and killed more than 2,000 people.

An ‘exponential increase’ in new cases is expected in the hardest-hit countries in coming weeks, UN health agency warned on Monday.

"As soon as a new Ebola treatment facility is opened, it immediately fills to overflowing with patients, pointing to a large but previously invisible caseload," WHO said in a statement about the situation in Liberia. "Many thousands of new cases are expected in Liberia over the coming three weeks."

So far, more than 3,500 people have been infected, nearly half of them in Liberia. The outbreak has taken a particularly heavy toll on health workers. The World Health Organization announced on Monday that one of its doctors working in Sierra Leone has been infected with Ebola.

In response to the spiralling disaster, US President Barack Obama said Sunday that the military would help to set up isolation units and provide security for public health workers responding to the outbreak.

Military personnel will set up a 25-bed field hospital in the Liberian capital, Col Steven Warren, a Pentagon spokesman, said on Monday. The clinic will be used to treat health care workers, a high number of whom have become infected in this outbreak.

Once set up, the centre will be turned over to the Liberian government. There is no plan to staff it with US military personnel, Warren said. Liberia welcomed the news.

"This is not Liberia's particular fight; it is a fight that the international community must engage very, very seriously and bring all possible resources to bear," said Information Minister Lewis Brown.

In addition, Britain will open a 62-bed treatment centre in Sierra Leone in the coming weeks. It will be operated by military engineers and medical staff with help from the charity Save the Children, Britain's Department for International Development said on Monday.

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