The director general of the World Health Organization said Sunday he believed a swift end could be put to the outbreak of Ebola in northwestern DR Congo, some 21 days since the outbreak that has left 27 people dead over the past month.

"I am cautiously optimistic that we shall be able to bring it to an end soon," Dr. Tedros Adhanom Ghebreyesus told reporters at Kinshasa airport.

Earlier, he had tweeted that "it's far too early to declare victory in the #Ebola outbreak. But the signals are positive and we are cautiously optimistic."

His comments come after WHO and NGOs began helping the Democratic Republic of Congo immunise front-line workers with a vaccine that has shown to be highly effective in trials, though it still awaits regulatory approval.

A total of 1,369 people have received the experimental vaccine since May 21, the ministry said last Wednesday.

Tedros is on an "evaluation" visit to the country as WHO monitors the progress of surveillance teams of epidemiologists working to assess the situation after local authorities declared the outbreak in a remote part of the province of Equateur on May 8.

On Thursday WHO gave a total of 59 confirmed, probable and suspected Ebola cases, of which 27 people had died, with one new case confirmed on June 6.

Tedros noted the cases were in "places which are not very accessible" and that the epidemiologists were going from village to village as they battled the logistical difficulties of covering ground as swiftly as they could.

The outbreak is DR Congo's ninth since the first in 1976.