DAKAR, Senegal -- More than 10,000 people have been infected with Ebola and nearly half of them have died, according to figures released Saturday by the World Health Organization, as the outbreak continues to spread.

The Ebola epidemic in West Africa is the largest ever outbreak of the disease with a rapidly rising death toll in Guinea, Liberia and Sierra Leone. There have also been cases in three other West African countries, Spain and the United States.

The U.N. health agency said Saturday that the number of confirmed, probable and suspected cases has risen to 10,141. Of those cases, 4,922 people have died. Its figures show about 200 new cases since the last report, four days ago.

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Even those grisly tolls are likely an underestimate, WHO has warned, as many people in the hardest hit countries have been unable or too frightened to seek medical care.

A shortage of labs capable of handling potentially infected blood samples has also made it difficult to track the outbreak. For example, the latest numbers show no change in Liberia's case toll, suggesting the numbers may be lagging behind reality.

On Thursday, authorities confirmed that the disease had spread to Mali, the sixth West African country affected, and on the same day a new case was confirmed in New York, in a doctor recently returned from Guinea.

Mali had long been considered highly vulnerable to the disease, since it shares a border with Guinea. The disease arrived there in a 2-year-old, who traveled from Guinea with her grandmother by bus and died Friday.

The toddler, who was bleeding from her nose during the journey, may have had high-risk contact with many people, the World Health Organization warned. So far, 43 people are being monitored in isolation for signs of the disease, and WHO said Saturday that authorities are continuing to look for more people at risk.

To help fight Ebola, the U.N. humanitarian flight service airlifted about 1 ton of medical supplies to Mali late Friday. The seats of the plane were removed to make room for the cargo, which included hazard suits for health workers, surgical gloves, face shields and buckets, according to the World Food Program, which runs the flights.

The spread of Ebola to Mali has highlighted how easily the virus can jump borders, and Malian border police said that neighboring Mauritania closed its border with Mali.

The health minister of Ivory Coast, which borders Guinea and Mali, said authorities there were looking for a nurse who may have Ebola and fled from Guinea, where he was being monitored by officials. But Raymonde Goudou stressed that it was still not clear whether the man had Ebola.

There was concern also in Ghana, where some worried a strike by health care workers could leave the country vulnerable to the disease. Ghana does not border any country with reported cases, but it is serving as the headquarters for the U.N. mission on Ebola.

In Liberia, the country hardest hit by the epidemic, U.S. forces have been building desperately needed treatment centers and helping to bring in aid. On Saturday, Maj. Gen. Darryl Williams, who was in charge of the troops assigned to the Ebola response, handed power to Maj. Gen. Gary J. Volesky, the 101st Airborne commander.

"I've been told that by a number of people that the task we face is extremely hard. Well, a fairly famous person once said hard is not impossible," Volesky said. "Together, we're going to beat it."