The World Health Organization (WHO) has raised the risk of the Bundibugyo strain of Ebola turning into a national outbreak in the Democratic Republic of Congo (DRC) to "very high".

The strain, for which there is no approved vaccine or treatment, was declared an emergency ​of international concern by the WHO on ​Sunday. 

"We are now revising our risk assessment to very high at the national level, high at the regional level, and low at global level," WHO Chief Tedros Adhanom Ghebreyesus told reporters.

So far, 82 cases have been confirmed in Congo, with seven confirmed deaths, 177 suspected deaths and almost 750 suspected cases. The situation in Uganda is stable, with two cases confirmed in people who travelled from the DRC, one of them fatal, Tedros said.

"The potential of this virus spreading rapidly is high, very high, and that changed the whole dynamic," said Abdirahman Mahamud, WHO Director of Health Emergency Alert & Response Operations. 

Measures taken in Uganda, including intense contact tracing and cancellation of a mass gathering, appear to have been effective in stemming the spread of the virus, Tedros said. 

A US national who was working in Congo has been confirmed positive and transferred to Germany for care.

"We are also aware of reports today about another American national with a high-risk contact who has been transferred to the Czech Republic," Tedros added.

The WHO's chief scientist, Sylvie Briand, said an antiviral treatment called Obeldesivir could be used among Ebola contacts to prevent them developing the disease.

Obeldesivir is an experimental oral Covid antiviral drug from Gilead Sciences.

"This is a promising treatment drug, but it has still to be implemented under a very, very strict protocol," Briand said.