MONROVIA, Liberia -- The United States and Britain will send medical equipment and military personnel to help contain West Africa's Ebola outbreak, as the World Health Organization warned Monday that many thousands of new infections are expected in Liberia in the coming weeks.

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, the UN health agency warned.

"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 Monday that one of its doctors working in Sierra Leone has been infected with Ebola.

In response to the spiraling disaster, U.S. 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 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 U.S. 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 Monday.

The clinic will also include a special section for treating health care workers, offering them high-quality, specialist care, the statement said.

Currently, there are about 570 beds in Ebola treatment centres in Guinea, Sierra Leone and Liberia, the hardest-hit countries, and the World Health Organization says nearly 1,000 more are needed, the vast majority of those in Liberia.

Doctors Without Borders welcomed both the American and British announcements, but warned even the latest surge in efforts may not be enough, saying the disease was moving "catastrophically through the population much faster than new facilities are being created."

And experts say it's not just beds, but that more international and local health workers that are needed. Doctors Without Borders also urged Washington to not simply set up clinics but also to staff them.

Many health workers, however, have been reluctant to respond to the crisis out of concern that there isn't enough protective equipment to keep them safe.

Ebola is spread through the bodily fluids of people who show symptoms, and doctors and nurses are at high risk of infection because they work closely with the sick. The WHO doctor whose infection was announced Monday is the second health care worker with the agency to catch Ebola. The doctor is in stable condition and will shortly be evacuated, the agency said.

In Liberia alone, 152 health care workers have been infected with Ebola and 79 have died, WHO said, noting that country had too few doctors and nurses even before the crisis.

"Every infection or death of a doctor or nurse depletes response capacity significantly," it said.

UN Secretary-General Ban Ki-moon called several world leaders over the weekend, including the British prime minister and French president, to urge them to send more medical teams and money to fight the outbreak.

Officials have said flight bans and border closures -- meant to stop the disease's spread -- are slowing the flow of aid and protective gear for doctors and nurses to the region.

At an emergency African Union meeting Monday, members agreed to open borders that have been closed and lift bans on flights to and from affected countries, according to Nkosazana Dlamini Zuma, chair of the AU's Commission. But it was unclear how quickly those promises would be kept.

Earlier, Senegal, which has shut its borders and blocked flights, said it was planning to open a "humanitarian corridor" to the affected countries.