Tedros Adhanom Ghebreyesus told reporters the outbreak in the Democratic Republic of the Congo now ranks as the third-largest on record.
Health officials confirmed more than 2,000 cases and 796 deaths since May 15.
The virus first appeared in Ituri Province. It then moved into other eastern provinces and crossed into Uganda.
Response teams work to trace contacts and isolate patients, but many new cases still come from unknown chains.
Experts note the outbreak grew quicker in the last month than in any earlier Ebola event.
DRC officials lead the effort with support from WHO and partners.
They started clinical trials for treatments to improve survival rates.
Health workers continue to set up treatment centers while communities receive updates on prevention steps.
Officials say the next weeks will show whether the response can match the pace of the virus.
