An outbreak of the deadly ebola virus has spread from the central African nation of Gabon to neighboring Republic of Congo, the World Health Organization said Thursday.

Fifteen people have died in Gabon since Ebola was first reported earlier this month. Eleven cases have now been detected in Republic of Congo, the U.N. health agency said in a statement.

Health authorities had feared the disease would spread as the Gabon outbreak was just a few miles from the border of Republic of Congo.

At least one woman believed to be infected with Ebola fled Gabon and was located in the village of Mbomo, Republic of Congo, health authorities said Wednesday. The woman's baby had died and she was reported to be in a serious condition.

"The international team is operational on both sides of the border and is working closely in the field with the team from the Congolese Ministry of Health on outbreak control interventions," WHO said.

Medical staff are trying to trace everyone who has had contact with any of the known victims and observing them for signs of the virus. A total of 133 contacts are being followed up in Gabon and 94 in the Republic of Congo, WHO said.

Ebola is one of the most deadly viral diseases known to humankind, causing death in 50 to 90 percent of those who become infected.

The virus is passed through contact with bodily fluids, such as mucus, saliva and blood, but is not airborne. It incubates for four to 10 days before flu-like symptoms set in. Eventually, the virus causes severe internal bleeding, vomiting and diarrhea.

There is no cure, but the disease usually kills its victims faster than it can spread, burning out before it can reach too far.

WHO says over 800 people have died of the disease since the virus was first identified in 1976 in western Sudan and in a nearby region of Congo. The disease last struck in Uganda, killing 224 people last year.