Sixty-three confirmed and probable cases have been reported in the Ebola outbreak in Uganda, including 29 deaths, according to the World Health Organization.
WHO chief Tedros Adhanom Ghebreyesus said the vaccines used to curb recent outbreaks in the neighbouring Democratic Republic of Congo were not effective against the type of Ebola virus circulating in Uganda.
And he said the Uganda outbreak was taking a toll on health workers.
"So far, 63 confirmed and probable cases have been reported, including 29 deaths," Tedros told a press conference in Geneva on Wednesday.
"Ten health workers have been infected and four have died. Four people have recovered and are receiving follow-up care."
He said the WHO was supporting the Ugandan government in its response to the outbreak, which has been reported in four districts.
The UN's health agency has released $2 million from its contingency fund for emergencies and is working with partners to support the health ministry by sending additional specialists, supplies and resources, Tedros said.
"When there is a delay in detecting an Ebola outbreak, it's normal for cases to increase steadily at the beginning and then decrease as life-saving interventions and outbreak control measures are implemented," he said.
"The vaccines used successfully to curb recent Ebola outbreaks in the Democratic Republic of Congo are not effective against the type of Ebola virus that's responsible for this outbreak in Uganda.
"However, several vaccines are in various stages of development against this virus, two of which could begin clinical trials in Uganda in the coming weeks, pending regulatory and ethics approvals from the Ugandan government."