The Ebola outbreak declared in May has killed more than 1,000 people in the Democratic Republic of Congo and Uganda, the World Health Organization said Wednesday.

In all, 2,473 cases of the deadly virus, including 999 deaths, have been confirmed in DRC, while 20 cases, including two deaths have been registered in neighbouring Uganda, according to a WHO overview based on official data from the two countries.

The DRC's 17th Ebola outbreak was declared on May 15 after several deaths in Ituri, and has remained concentrated in the mineral-rich northeastern province, plagued by armed groups.

Cases of Ebola, which spreads through close contact and infected bodily fluids, have also been found in four other DRC provinces.

The cases found in Uganda were mostly Congolese citizens who crossed the border, and the country says it has gone three weeks without registering any new cases.

WHO chief Tedros Adhanom Ghebreyesus warned last week that the outbreak had over the past month "expanded faster than any previous outbreak" of Ebola.

Despite a ramping up of the response, "the outbreak remains ahead of us and we are still in a phase of catching up", Thierno Balde, WHO's incident manager for Bundibugyo virus in DRC, told reporters on Tuesday.

There is currently no approved vaccine or treatment for the Bundibugyo strain behind this outbreak.

But the WHO has reported "encouraging progress", with trials for two potential treatments, a vaccine and a post-exposure prophylaxis underway on the ground.