"Strong progress" has been made in calming Congo's deadly Ebola outbreak in a city of 1.2 million and in the rural outpost where the epidemic was declared one month ago, the World Health Organisation has said.

The focus now turns to "some of the most remote territory on Earth", officials at the agency said.

WHO chiefs expressed cautious optimism as the pace of new cases slowed. Congo's health ministry announced a new confirmed Ebola case late on Thursday, bringing the total to 38, including 13 deaths.

The new case is in the remote Iboko health zone in Congo's north-west. Health workers also have been chasing contacts of those infected in Mbandaka city, a provincial capital on the heavily travelled Congo River, and in Bikoro town, where the outbreak was declared.

While Ebola's spread to a major city has complicated efforts to track all contacts of those infected, the presence of the virus in Iboko poses another set of problems.

The forested terrain is so rough that even four-wheel-drive vehicles cannot reach the area, which has no electricity, WHO's emergency response chief Peter Salama told reporters in Geneva.

Motorcycles are only now arriving and health workers are sleeping 15 to 20 people to a tent.

"This is a major logistical and boots-on-the-ground epidemiological endeavor now," Mr Salama said, adding that work there will go on for weeks.

The WHO has vaccinated more than 1,000 people over the past two weeks in all areas of the outbreak, including health workers who are at high risk. The virus spreads via bodily fluids of infected people, including the dead.