The Ebola outbreak in the Democratic Republic of Congo has spread to a city surrounded by rebels, creating challenges that aid workers were "dreading," the World Health Organization (WHO) said Friday.

One probable case and one suspected case have emerged in the city of Oicha in DRC's North Kivu province, WHO's emergency response chief, Peter Salama, told reporters. While Oicha itself is not under rebel control, Salama said the city is entirely surrounded by territory held by a feared Ugandan insurgent group known as the Allied Democratic Forces (ADF).

"For the first time we have a confirmed case in an area with high insecurity," Salama said. "It really was the problem we were anticipating and the problem at the same time we were dreading."

The outbreak, the 10th to hit DRC since 1976, began on Aug. 1 in the North Kivu town of Mangina. The virus has killed 63 people so far, out of 103 confirmed or probable cases, but Salama warned health workers were expecting "at least one additional wave of cases."

After its widely-criticized handling of the 2013-2016 West Africa Ebola epidemic that killed 11,300 people, the WHO pledged to revamp its response systems. The U.N. agency was widely praised for quickly containing an outbreak in DRC's northern Equateur province earlier this year, but said from the outset that restive North Kivu in the east of the country posed additional challenges.