It is the first weekly increase in 2015, ending a series of encouraging declines.
The WHO says Sierra Leone registered 80 of the 124 new cases, Guinea 39 and Liberia the remaining five. Almost 9,000 people have died from Ebola since December 2013.
Dr David Nabarro, the United Nations special envoy on Ebola, said the small rise in Ebola cases was a concern but they knew there would be flare-ups.
"We're on a good path, this is coming down, but we really have to be vigilant because there are still pockets of infection," he said.
