Ebola continues to spread in two "troublesome areas" in  Sierra Leone and Guinea's interior, a senior UN official says.

The outbreak is "still flaming strongly" in western Sierra Leone and some parts of the forested interior of Guinea, the UN’s special envoy for Ebola, David Nabarro, told a news conference in Geneva on Tuesday.

"There are two particularly troublesome areas," with high levels of transmission, Nabarro said.

The increase in transmission in western Sierra Leone reflects how communities there haven’t yet fully acted to avoid infection themselves. There's also a lack of fully staffed treatment centres and places to keep those who are sick away from others, he said.

The northern part of Guinea’s interior is the second trouble spot.

"We have been working very closely with Mali to try to make sure if cases perchance cross the border that they can be dealt with very quickly."

The Ebola outbreak has infected more than 17,800 people, the vast majority in Liberia, Guinea and Sierra Leone. Of those, around 6,300 have died.

Meanwhile, Liberia’s president said Ebola hot spots remain in her country, many in rural areas.

Liberian President Ellen Johnson Sirleaf launched "Operation Ebola Must Go," on Monday to rid Ebola from the country.

"The disease has retreated into places that are hard to reach," Sirleaf said in remarks broadcast Tuesday on state radio from the launch in the Monrovia township of New Georgia. "We all have to intensify our effort to travel that difficult, very difficult last mile."

With files from Reuters and The Associated Press