The U.N. Ebola chief said Monday he believes “it’s only a matter of weeks” before the Ebola outbreak in Sierra Leone ends, but stopping the deadly disease in Guinea will take more time because in some areas unsafe traditions still hold sway over what’s good for people’s health.

Dr. David Nabarro stressed in an interview with The Associated Press on Monday that the Ebola epidemic “absolutely isn’t over” though Liberia has been Ebola free since May 9.

“We have cases occurring, not in huge numbers but enough to be very disturbing, in Sierra Leone and in Guinea,” he said, and the situation in Guinea is especially troubling because surprise cases have been popping up that are not part of known chains of transmission.

“Getting on top of this is difficult,” Nabarro said. “It’s like looking for needles in haystacks – very hard to find the people who are ill and get them under treatment quickly before they perhaps infect others.”

Since the Ebola outbreak was first reported in March 2014, the World Health Organization said 27,049 cases and 11,149 deaths have been reported, almost all in the three West African countries.

According to the latest WHO figures, in the week ending May 24 there were 12 new cases of Ebola – 9 in Guinea and 3 in Sierra Leone, down from a total of 35 cases the previous week.

Nabarro is heading to Guinea on Thursday to review efforts to reduce the number of new cases in the country and neighboring Sierra Leone to “zero.”

In Sierra Leone, he said, there are three districts with a few cases being reported – the capital Freetown, nearby Port Loko, and the Kambia district further west which borders Guinea.

“The general trendlines are that increasingly, new cases are known contacts of people who’ve already got the disease, and also when cases are detected they’re brought under treatment quickly,” Nabarro said.

“And that means that we believe that it’s only a matter of weeks before the outbreak is ended in Sierra Leone,” he said, “and that’s because of strong participation of communities, a very coordinated national response, and strong leadership from Ernest (Bai) Koroma, the president.”

Guinea, on the other hand, has chains of transmission which health workers don’t fully understand and communities that aren’t cooperating with health workers trying to eradicate Ebola because they view them as trying to disrupt their traditional way of life, especially in dealing with illness and death, Nabarro said.

Touching the body of an Ebola victim can transmit the disease, but traditional burials in Guinea are hands on – and determine whether the spirit of the dead person will move into others in the community in a happy way or will be disturbed and take it out on people, Nabarro said.

This is the issue that experts are now focusing on, and they’ve brought in anthropologists, local religious leaders, particularly imams, and local political leaders to help build confidence between health responders and the impacted communities, he said.

“We need to find a compromise, a dignified, traditional approach to burial but also safety, so we don’t get transmission of disease, and it’s that interface that’s at the center of the current effort to end the outbreak in Guinea,” Nabarro said.

“It can be done. I’m utterly confident it will be done. I just can’t tell you how long it will take,” he said.

Nabarro stressed that Ebola is now confined to a small belt along Guinea’s coast, noting that the forest region where the outbreak started has been Ebola free for weeks.

He wouldn’t predict when West Africa will be Ebola free.

“If we can maintain attention, focus and most important sensitivity to the concerns and anxieties of the communities, then we will be able to end the outbreak in the foreseeable future, but I’m not going to put a time on it,” Nabarro said.