Russian-backed Ukrainian rebels said on October 23 they had banned the Doctors Without Borders (MSF) charity group and United Nations agencies from operating in Donetsk.
MSF provides crucial medical care for patients in the separatist-held area, especially those suffering from diabetes and kidney failure, as well as operating an anti-tuberculosis program in Donetsk prisons.
A representative of the self-proclaimed Donetsk People's Republic told AFP: "I can confirm the decision to strip its (MSF's) accreditation."
MSF and nine other UN agencies and charity groups were kicked out of the neighboring rebel region of Luhansk in late September for not having required accreditation.
MSF director of operations Bart Janssens called the latest decision disturbing and "vague," and said the group was "extremely worried."
"MSF is the largest player in the region and now we will have to stop people's treatment," Janssens told AFP.
UN agencies such as the World Food Program were also banned as they did not have the necessary accreditation.
The rebels said that they had spared the International Committee of the Red Cross and the Czech Republic's People in Need -- two other earlier targets of alleged violations of accreditation requirements.
The UN food program would still be able to carry out some of its functions through the People in Need group, they said.
In Luhansk, separatists accused MSF of "illegally storing psychotropic medication" that lacked proper registration in either Russia or Ukraine.
MSF denies the allegation.