The Ebola outbreak in Guinea and Sierra Leone is expected to take all of 2015 to stamp out and may persist even longer because of dwindling financing, the World Health Organization (WHO) warned on Tuesday. 

Guinea and Sierra Leone reported a total of 12 cases in the week to Sunday May 24th, down from 36 the previous week. They included new infections and deaths that occurred outside of treatment centers as communities hide their sick, it said.

Liberia, the third country hit by West Africa's year-long epidemic, was declared Ebola-free on May 9 after 42 days without a case. The disease has killed more than 11,100 people overall among 27,000 infected.

"When you look at the case numbers today (in Guinea and Sierra Leone), this is where Liberia was in January. As you know, it took Liberia four months to get from those numbers to zero," WHO Special Representative for Ebola Dr. Bruce Aylward told a news briefing.

"In a best-case scenario, one would be looking to perhaps stopping transmission by the end of September. But we're not dealing with the best-case scenario," he said.

The rainy season has begun in West Africa, compounding the difficulty of reaching remote areas, and one in three new infections still occurs in people not suspected of having been exposed to the virus, he said.

"So the situation is very different; it is not a best-case scenario. Which means end-September is not a reasonable planning time frame. We've got to plan right out through the end of 2015," Aylward said.

The virus is now concentrated in coastal areas with "the last battlefield" the Forecariah district in Guinea and densely populated slum areas near Freetown in Sierra Leone, he said.

But maintaining more than 1,000 WHO staff on the ground and a vast aid operation is expensive, especially the World Food Programme's air bridge, he said.

"It is going to cost $50 million over the next six months to keep those helicopters in the air, to keep the planes in the air and to keep those operations working," Aylward said.

The WHO lacks more than $100 million towards its $350 million budget for its Ebola operation over the next six months, jeopardizing its ability to keep people on the ground, he said.

"There is no reason that Ebola cannot be beaten, but financing is increasingly becoming the most glaring potential reason for failure," he said.