At least 166 people have died and WHO says it is aware of 372 other cases.
International agencies have begun airlifting emergency medical supplies and setting up isolation tents to try to contain the outbreak.
Ebola is highly contagious. People contracting the disease suffer severe stomach pain and internal bleeding.
Medecins sans Frontieres has reinforced its medical personnel and has flown three tonnes of supplies to the provincial capital Kananga, to be distributed in the affected areas.
The supplies include tents and plastic sheeting to build isolation facilities. Medicines, water and sanitation materials are also being sent.
"Ebola kits" have been provided for the medical teams - they include protective gloves, boots and uniforms which are designed to be destroyed after use.
Local health authorities are helping to disinfect contaminated areas.
The fatality rate for Ebola, which has no known cure, is as high as 90%.
Specialist laboratories in Gabon and Atlanta in the US confirmed Ebola from blood samples, saying they also showed the presence of Shigella dysentery - which is hampering efforts to identify Ebola victims.
WHO has also requested additional support from the global outbreak alert and response network, and says specialised laboratories in Gabon, Canada and the US will share the analysis.
They say there could be a "possible concurrent outbreak of another etiology".
It is three months since people started falling sick from a mystery virus in several villages around Kananga, the capital of West Kasai region.
Several villages are under quarantine, but WHO says so far there is no need for any further restrictions on travel or trade with DR Congo.
The incident is the worst for several years and is likely to have serious consequences for some time to come - even if the spread has been contained.
It is thought to be transmitted through the consumption of infected bush meat and can also be spread by contact with the blood secretions of infected people.
DR Congo's last major Ebola outbreak killed more than 200 people in 1995 in Kikwit, about 400km (250 miles) west of the current outbreak.
The last major incidence of the disease was in Uganda in 2001 when more than 400 cases were reported and more than half of the patients died.
Uganda has issued a red alert to border posts neighbouring the DR Congo and has instructed staff at Entebbe international airport to be on the lookout for passengers who show symptoms of fever.