A World Health Organization (WHO) official said on Tuesday it was still possible to bring under control an Ebola outbreak that is outpacing containment efforts and has become the second-deadliest on record.

The global health agency said earlier this month that reversing Ebola's spread would be possible within a three-month period, but since then there have been signs that it has worsened, expanding to a sixth province.

"About the next plan for the three months, yes, if you have the necessary resources, I think it will be possible to do that," Thierno Balde, WHO incident manager, told reporters in Geneva from Bunia, in the Democratic Republic of Congo. 

The outbreak, Congo's worst yet of 17, has so far killed at least 2 325 people.

Resources are currently lagging requirements, and the WHO has raised about 60% of the $115-million it is seeking for the Ebola response.

Other challenges include delayed detection, overwhelmed surveillance teams, armed conflict and a lack of vaccines and treatments for the rare Bundibugyo species of the virus. 

Still, containment efforts are ramping up and an extra 400 beds have been added in the past two weeks and more epidemiologists will be deployed to trace contacts of the sick, Balde said. 

"The situation is quite difficult, but people are working here day and night trying to control this outbreak," he told reporters.

Other leading officials told Reuters that the outbreak was a long way from being under control, pointing out that between 70-80% of new cases have no known link to patients, meaning the virus is still spreading unchecked in many areas.  

"We are far from discovering the magnitude of this outbreak," said Dr Wessam Mankoula, Africa CDC's head of emergency preparedness and response. 

"It will not be over in a few weeks or even months," he said. 

The WHO Emergency Committee meets on Tuesday on Ebola, which was deemed a public health emergency of international concern, or PHEIC, by the UN agency on May 17. The body will decide whether to extend the emergency and give states recommendations for controlling its spread.