JOHANNESBURG — The World Health Organization says Congo’s deadly Ebola outbreak is now the second largest in history, behind the devastating West Africa outbreak that killed thousands a few years ago.

WHO emergencies chief Dr. Peter Salama late Thursday called it “a sad toll” as Congo’s health ministry announced the number of cases has reached 426. That includes 379 confirmed and 47 probable cases, with 242 deaths.

Attacks by rebel groups and open hostility by some wary locals have posed serious challenges that Ebola workers say they have never faced before. Many venture out on critical virus containment work only with the accompaniment of U.N. peacekeepers while gunfire echoes daily.

Salama predicted this month that the outbreak in northeastern Congo will last at least another six months before it can be contained.

The West Africa Ebola outbreak killed more than 11,000 people from 2014 to 2016. An outbreak in 2000 in Uganda sickened 425 people and killed 224 of them and it was, until now, the second-worst outbreak.

Day by day, reports by health organizations note one new difficulty after another for the Ebola outbreak in Congo, even as their work sets milestones that have given new hope in the fight against one of the world’s most notorious diseases.

More than 37,000 people have received Ebola vaccinations, and Congo has begun the first-ever trial to test the effectiveness and safety of four experimental Ebola drugs. And yet the risk of Ebola spreading in so-called “red zones” — areas that are virtually inaccessible because of the threat of rebel groups — is a major concern in containing this outbreak.

“This tragic milestone clearly demonstrates the complexity and severity of the outbreak. While the numbers are far from those from West Africa in 2014, we’re witnessing how the dynamics of conflict pose a different kind of threat,” said Michelle Gayer, senior director of emergency health at the International Rescue Committee.

The alarmingly high number of infected newborns in Congo is another concern, and so far a mystery. In a separate statement on Thursday, WHO said so far in this outbreak, 36 Ebola cases have been reported among newborn babies and children under 2.

This is the first time this turbulent part of northeastern Congo has had an Ebola outbreak. Congo’s health ministry has carried vivid accounts of residents, spurred by rumors, who have been trying to stop safe burial practices that halt the spread of Ebola from victims to relatives and friends.

On Thursday, the ministry said a group of youths broke into a morgue, stole the body of an Ebola victim and returned it to their family.