Ebola is still spreading quickly in western Sierra Leone and deep in the forested interior of Guinea and more foreign health workers are needed to help tackle the epidemic, a senior U.N. official said on Tuesday.

The death toll from the Ebola outbreak in West Africa has risen to 6,331 in the three worst hit countries, with Sierra Leone overtaking Liberia as the country with the highest number of cases, World Health Organization figures showed on Monday.

"We know the outbreak is still flaming strongly in western Sierra Leone and some parts of the interior of Guinea. We can't rest, we still have to push on," said David Nabarro, the U.N. Secretary General's special envoy for Ebola.

Treatment centers are still opening in Sierra Leone and need expert staff, he told a news briefing in Geneva.

The deadly virus is spreading particularly in Sierra Leone's capital of Freetown and Port Loko "where there is a need for a much more intense response", said Nabarro, a veteran public health expert.

"The increase in transmission in western Sierra Leone is a reflection of the fact that communities there have yet to fully embrace the outbreak and to take action to avoid infection themselves," he added.

The second "troublesome" area is the northern part of Guinea's interior, a region known as Guinea Forestiere, he said.

"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."

"We do need to keep making it clear that people are needed and needed urgently ... Every day we wake up and realize the enormity of the job ahead," Nabarro said.