The Democratic Republic of Congo's (DRC's) Ebola outbreak is spreading at its fastest rate yet, eight months after it was first detected, the World Health Organization (WHO) said on Monday.

Each of the past two weeks has registered a record number of new cases, marking a sharp setback for efforts to respond to the second biggest outbreak ever, as militia violence and community resistance have impeded access to affected areas.

Less than three weeks ago, the WHO said the outbreak of the haemorrhagic fever was largely contained and could be stopped by September, noting that weekly case numbers had halved from earlier in the year to about 25.

But the number of cases hit a record 57 the following week, and then jumped to 72 last week, said WHO spokesperson Christian Lindmeier. Previous spikes of around 50 cases per week were documented in late January and mid-November.

More alarmingly, more than half of the Ebola deaths last week occurred outside of treatment centres, according to Congo health ministry data, meaning there is a much greater chance they transmitted the virus to those around them.

"People are becoming infected without access to response measures," Lindmeier told Reuters.

The current outbreak is believed to have killed 676 people and infected 406 others. Another 331 patients have recovered.

In the past two months, five Ebola centres have been attacked, some by armed militiamen. That led French medical charity Medecins Sans Frontieres (MSF) to suspend its activities in two of the most affected areas.

Another challenge has been a mistrust of first responders. A survey conducted last September by medical journal The Lancet found that a quarter of people sampled in two Ebola hotspots did not believe the disease was real.

Lindmeier said new approaches to community outreach were showing signs of progress and that some previously hostile local residents had recently agreed to grant health workers access.

One treatment centre that closed in February after being torched by unknown assailants reopened last week.

More than 11 000 people died in West Africa's 2013-16 Ebola outbreak. Since then, health authorities have worked to speed up their responses and deployed an experimental vaccine and treatments, both of which have been considered effective.