MONROVIA, Liberia -- American mobile Ebola labs should be up and running in Liberia this week, and U.S. troops have broken ground for a field hospital, as the international community races to increase the ability to care for the spiraling number of people infected with the disease.

Ebola is believed to have sickened more than 6,500 people in West Africa, with the vast majority of the cases in Liberia, Guinea and Sierra Leone. More than 3,000 deaths have been linked to the disease, according to the World Health Organization.

But even those tolls are likely underestimates, partially because there aren't enough labs to test people for Ebola. The numbers for Liberia, in particular, have lagged behind reality because it takes so long to get test results, WHO has warned.

Nigeria has also recorded some cases, and Senegal had one; in both cases, a sick traveller imported the disease. Both countries moved quickly to isolate the sick and trace anyone who had contact with them, and neither country has had a new case for weeks.

"It's reasonable to assume there will be no more transmission in those countries," Steve Monroe, deputy director of the National Center for Emerging and Zoonotic Infectious Diseases at the U.S. Centers for Disease Control and Prevention, told reporters Tuesday.

By contrast, in the worst affected countries, the disease has whipped through entire communities, killing whole families because there are too few doctors and nurses and not enough beds to isolate and treat the sick. At least 3,700 children have lost a parent in the outbreak, the U.N. children's agency said Tuesday, and fear of infection has made it difficult to find people to care for them.

In response to the accelerating outbreak, the United Nations has created its first ever mission for a public health emergency.

"The risk of expansion is dramatic and the number of affected people is doubling," Anthony Banbury, the head of the U.N. mission, told reporters Tuesday in Ghana, where the mission is based.

Over the next month, the mission will work on getting the necessary infrastructure, including treatment centres, into the field, he said.

Aid agencies and many countries are also pouring in supplies and equipment.

Two mobile Ebola labs staffed by U.S. Naval researchers arrived this weekend and will be operational this week, the U.S. Embassy in Monrovia said in a statement Monday. The labs will reduce the amount of time it takes to learn if a patient has Ebola from several days to a few hours.

The U.S. military also delivered equipment to build a field hospital, originally designed to treat troops in combat zones. The 25-bed clinic will be staffed by American health workers from the U.S. Public Health Service and will treat doctors and nurses who have become infected.

Ebola is transmitted through contact with bodily fluids, so health care workers are at high risk of infection. They have become sick at an alarming rate in this outbreak, WHO says, with 375 infected so far.

The U.S. is planning to build 17 other clinics in Liberia and will help to train more health workers to staff them. Britain has promised to help set up 700 treatment beds in Sierra Leone, and its military will build and staff a hospital in that country. France is sending a field hospital and doctors to Guinea.

But the needs remain enormous. The World Food Program said Tuesday it only has about 40 per cent of the $93 million it needs to deliver food to people who are struggling to feed themselves because their neighbourhoods have been quarantined or they've lost the heads of their households. Around 1,500 treatment beds have been built or are in the works, but that still leaves a gap of more than 2,100 beds, says WHO. Between 1,000 and 2,000 international health care workers are needed, according to the agency.

- AP medical writer Lauran Neergaard in Washington and Associated Press writer Francis Kokutse in Accra, Ghana, contributed to this report.