On Wednesday Adamou Yada, WHO's representative in Congo, which has faced serious outbreaks of the disease in the past, said: "The results (of laboratory tests) came in yesterday ... It is indeed a case of Ebola."
The latest outbreak is in the forested northwestern Cuvette-Ouest region, where nearly 150 people died from Ebola in 2003.
Neighbouring Gabon also had outbreaks in 2001 and 2002.
"Since the beginning (of the outbreak), we have registered 11 cases, including nine deaths," Yada said.
There is no known cure for Ebola, which is passed on by infected body fluids and kills between 50 and 90% of victims, depending on the strain.
Ebola damages blood vessels and can cause extensive bleeding, diarrhoea and shock. Its worst outbreak, in 1995, killed more than 250 people in the Democratic Republic of Congo.
Ebola's worst outbreak led to 250
deaths in DPR Congo in 1995
In a statement on its website, WHO said of the 11 cases in Congo, one had been confirmed as Ebola by laboratory tests and 10 were epidemiologically linked.
A total of 81 contacts were being monitored in the towns of Etoumbi and Mbomo, it said.
Officials from Congo's Health Ministry, WHO and Medecins Sans Frontieres-Holland are in the field, following up contacts and raising awareness about the disease, WHO said.
Scientists think past outbreaks in Cuvette-Ouest, near the border with Gabon, were caused by the consumption of infected monkey meat.
Bushmeat is a staple among forest communities in West and Central Africa and a delicacy in many cities.
Elephant hunters might have been behind the latest outbreak, they "must have handled a primate found dead in the forest" during a recent long hunt
WHO's representative in Congo
Yada said elephant hunters might have been behind the latest outbreak, saying they "must have handled a primate found dead in the forest" during a recent long hunt.
Rodriguez Abiabouti, an aid worker in Cuvette-Ouest, said most people in the remote region survive by hunting.
"All the outbreaks of Ebola in Cuvette-Ouest have been preceded by the unexpected discovery of an abnormally high mortality rate among animals, primates and antelopes," Abiabouti said.
Etoumbi has not been quarantined but movement into and out of the region is being monitored.
Information campaigns to warn people of the dangers of eating bush meat have been stepped up and food aid will be provided to local populations.
An outbreak in Angola of the Marburg virus, a close relative of Ebola, has killed 277 people, WHO said earlier this month.