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.
Only a week ago the WHO announced its lowest weekly tally of new cases since June 2014, raising hopes that a turning point in the battle against the disease might have been reached.
But suspicion of aid workers, especially in Guinea, and unsafe local practices were continuing to hamper efforts to contain the virus, the United Nations agency said.
Eleven new cases were blamed on one unsafe burial that took place in eastern Guinea on the border with Cote d’Ivoire, where a rapid response team has now been deployed, the WHO adds.
Mourners have caught the disease in the past by touching the highly-contagious bodies of dead loved ones.
Nearly one-third of Guinea’s 34 prefectures had reported at least one security incident or other form of refusal to cooperate with health workers in the previous week.
The WHO emphasised the need to step up efforts before the start of the April-May rainy season, when downpours can block roads and make it difficult for health teams to travel.