The seven days ending May 24 "saw the highest weekly total of confirmed cases of Ebola virus disease for over a month," the UN health body said in its latest update.
A full 35 new cases were reported during the week in Guinea and Sierra Leone, up from just nine a week earlier.
Guinea, where the outbreak began in late 2013, was hardest hit last week, with 27 new cases reported, compared to just seven the week before.
Eleven of those cases were reported in the western prefecture of Dubreka, and most of them appeared to be linked to people who attended the funeral of someone believed to have died from Ebola in mid-April.
Eleven other cases were reported in the prefecture of Forecariah, but it was the five remaining cases found in the north-western prefecture of Boke, which borders Guinea-Bissau, that had experts most concerned.
WHO said experts had been sent to the border to determine if possible cases may have crossed over, and ensure that anyone known to have been in contact with Ebola patients are tracked.
Sierra Leone, which had appeared to be heading in the same direction as neighboring Liberia, meanwhile saw the number of new cases shoot up to eight from just two a week earlier.
The increase in Sierra Leone brought to an end "a sequence of three consecutive falls in weekly case incidence," WHO said.
The UN agency also deplored that, for the first time in five week, a Sierra Leonean health worker had tested positive for Ebola.
The health worker had been working at an Ebola treatment center near Freetown the same facility where an Italian male nurse who tested positive for Ebola after returning home had worked.
Ebola, one of the deadliest viruses known to man, is spread only through direct contact with the bodily fluids of an infected person showing symptoms such as fever or vomiting.
People caring for the sick or handling the bodies of people infected with Ebola are thus especially exposed.
Since the beginning of the outbreak, 869 health workers have been confirmed to be carrying Ebola, and 507 of them have died, WHO said.
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