Unidentified attackers killed two Ebola health workers in eastern Congo over the weekend, the Health Ministry said, the latest in a string of assaults that have hampered efforts to stop the deadly spread of the virus.

Local mistrust of health officials and militia violence in Congo’s restive east have caused the number of new cases to surge. Nearly 2,500 people have been infected and more than 1,600 killed in the second biggest outbreak on record.

Dozens of responders have been injured or killed in attacks this year.

The two killed were community workers near Mukulia in North Kivu province, and had been receiving threats since December, with one attacked previously, the ministry said in a statement. Their names were not immediately released.

The first case of Ebola in the eastern Congo city of Goma was discovered on Sunday, raising concerns the virus could spread more quickly in a densely populated area close to the Rwandan border.

Goma, a lakeside city of one million people, is more than 350 km south of where the outbreak was first detected a year ago.

The patient was a priest who became infected during a visit to the town of Butembo, 200 km north of Goma, where he interacted with Ebola patients, Congo’s Health Ministry said in a statement.

He developed symptoms last week before taking a bus to Goma on Friday. When he arrived in Goma on Sunday, he went to a clinic, where he tested positive for Ebola, and was taken back to a clinic in Butembo on Monday.

“Due to the speed with which the patient has been identified and isolated, as well as the identification of all bus passengers from Butembo, the risk of spreading to the rest of the city of Goma remains low,” the ministry said.

Goma has been preparing for the arrival of Ebola for a year, setting up handwashing stations and making sure mototaxi drivers do not share helmets.

Ebola causes diarrhea, vomiting and hemorrhagic fever, and can be spread through bodily fluids. An epidemic between 2013 and 2016 killed more than 11,300 people in West Africa.