BUNIA, Congo (AP) — At least 500 people have died out of over 1,500 confirmed cases in Congo’s Ebola outbreak, authorities said, as frontline workers threatened to go on strike on Monday over unpaid benefits and poor working conditions.

The outbreak has recorded 1,561 cases, including 506 deaths, since it was declared on May 15 as the spread continues to outpace response, Congo's Ministry of Health said in its latest update on Sunday night.

Frontline workers deployed in Ituri province, the epicenter of the outbreak, issued a 24-hour notice on Sunday threatening to strike if authorities fail to pay them and improve their working conditions.

The workers include mostly health professionals who have been laboring with little rest as they battle attacks from angry residents and widespread skepticism about the virus.

In the notice to the government, a copy of which was seen by The Associated Press, the workers both in and outside hospitals said they had not been paid benefits since the outbreak began and they do not have adequate supplies for their work.

They also complained of poor salaries, the "arrogance" of teams sent from Congo's capital of Kinshasa, and the “excessive” use of labor from other provinces without prioritizing local labor in Ituri, as well as the lack of adequate equipment.

The strike threats come just days after enrollment for clinical trials started, raising concerns in the epicenter about its possible impact. Any strike could also hamper efforts to slow the spread of the outbreak, which is now confirmed in three eastern provinces including North Kivu and South Kivu.

The lack of approved vaccines or treatments for the Bundibugyo virus, which is responsible for the latest Ebola outbreak, has complicated response efforts. The more common Zaire virus, for which there is a vaccine, was responsible for most of Congo’s past 16 outbreaks of the disease.

Officials are yet to identify the outbreak’s patient zero and still need to trace possibly tens of thousands of people who have come in contact with infected individuals.

The first month of this Ebola outbreak was already the worst on record, the World Health Organization has said.