Sierra Leone restricted travel Thursday into and within three more "hotspots" of Ebola where 1.5 million people live, meaning that more than a third of the country's population is now under quarantine.

Sierra Leone is one of the hardest hit countries in an Ebola outbreak sweeping West Africa that is believed to have killed more than 2,900 people, according to World Health Organization tolls published Thursday. The agency says the situation in Sierra Leone continues to deteriorate, driven primarily by a sharp increase of cases in the capital, Freetown.

In an address to the nation on Wednesday night, President Ernest Bai Koroma put Port Loko, Bombali, and Moyamba districts under isolation, meaning only people delivering essential services can enter and circulate within these areas. WHO also noted that these districts were seeing a rise in cases.

In other areas, including in the capital, homes will be put under quarantine when cases are identified, a government statement after the address said.

The Ebola outbreak, the largest ever, has also hit Liberia and Guinea and is believed to have sickened more than 6,200 people. An Ebola patient also slipped into Senegal from Guinea but the disease does not appear to have spread there. Nigeria has linked 20 cases to the disease and eight deaths after a man infected with Ebola traveled there from Liberia; the outbreak seems to have been contained there as well.

"Nigeria is free of the virus now but we know that to be permanently free from it, we must remain vigilant and work with WHO and the international community to eradicate it completely from our sub-region and forestall the possibility of its re-emergence on our shores through migration," Nigerian President Goodluck Jonathan said Wednesday night.

The outbreak's unprecedented scale and geographic spread have pushed governments to impose severe measures, like the cordoning off of entire towns or regions. Last week, Sierra Leone imposed a three-day nationwide lockdown, confining its 6 million people to their homes while health teams spread out to look for the sick and educate people about the disease.

That exercise revealed that the outbreak is worse than thought, the government said, with 160 Ebola cases found during the shutdown.

"There is a desperate need to step up our response to this dreaded disease," the Sierra Leone government statement said. "The prognosis is that without additional interventions or changes in community behavior, the numbers will increase exponentially and the situation will rapidly deteriorate."

Two districts near the outbreak's epicenter -- Kenema and Kailahun -- had already been isolated. Liberia, the country hardest-hit by the disease, cordoned off areas of its capital at one point to slow transmission.