By Agence France-Presse
Doctor Tedros Adhanom Ghebreyesus, on a two-visit to DR Congo, said the suspected new case was in Bikoro, at the centre of the latest Ebola outbreak in the country's northwest, where so far 18 people have died.
It brings to 35 the number of reported cases, including two confirmed, according to the WHO tally.
The WHO is planning to begin a vaccination campaign in the area, near the border with the Republic of Congo, using an available stockpile of an experimental vaccine.
"The vaccines are going to arrive on Wednesday or Thursday. We have enough of them," said Tedros, without specifying the number of doses.
After meeting with DR Congo President Joseph Kabila in Kinshasa, the WHO chief praised "the government's very strong leadership" in responding to the health crisis and its "good coordination" with partners such as Unicef and Doctors without Borders (MSF).
On Friday the UN health agency had announced that it was preparing for all scenarios, including the "worst case scenario", in the latest Ebola outbreak in DR Congo.
Tedros said he hoped for a "better result" in this ninth Ebola outbreak in the vast central African country since 1976, when the deadly viral disease was first identified by a Belgian-led team.
Ebola is one of the world's most notorious diseases, being both highly infectious and extremely lethal.
The worst-ever Ebola outbreak started in December 2013 in southern Guinea before spreading to two neighbouring west African countries, Liberia and Sierra Leone, killing more than 11,300 people out of nearly 29,000 registered cases.