Nauru's government has ordered Doctors without Borders, also known as Medecins Sans Frontieres (MSF), to cease its work on the island, the medical aid group said in a statement on Saturday.
MSF was providing medical treatment to the asylum-seekers and locals suffering from mental health problems on the tiny Pacific island.
More than a thousand asylum-seekers, including children, are housed in Australian-funded refugee detention camps on Nauru under the county's controversial deal with Canberra to prevent people from reaching Australia.
"The Nauruan government informed Medecins Sans Frontieres that our services were 'no longer required' and requested that our activities cease within 24 hours," an MSF spokesman told AFP news agency.
According to the MSF website, the aid group has been providing psychological and psychiatric help on Nauru since late last year.
"The lack of mental-health support available affects both the Nauruan population and the asylum-seekers and refugees living on the island as part of the Australian government's policy of offshore processing," the organization said.
Worsening mental health
"Our teams have identified cases of schizophrenia and family violence, and concerning levels of depression in Nauru, especially among children," according to MSF.
In recent months, the health of many asylum-seeker children and teenagers — many of whom have been held on Nauru for much of their lives — has dramatically worsened, with the emergence of a serious and rare psychiatric disorder called "resignation syndrome."
The Asylum Seeker Resource Center (ASRC) estimates at least 30 children residing on Nauru are suffering from the syndrome.
A 2016 UN report pointed to "inhuman and degrading treatment" of minors in detention camps, "including physical, psychological and sexual abuse."
Is offshore detention necessary?
MSF said the charity group was "extremely concerned that the health of our patients may be affected by this decision and urge the authorities to grant us permission to continue our lifesaving work".
There was no immediate comment on the MSF statement from the Nauru government.
Canberra denies mistreatment of asylum-seekers in its detention centers on Nauru and Manus islands, insisting that offshore processing is necessary to protect people who try to reach Australian shores on rickety boats.
Many of these asylum-seekers come from war-torn countries like Syria and Iraq, but others also come from Africa and parts of Asia. But regardless of their origin, they are told that even if their asylum claims are found to be legitimate, the best they can hope for is resettlement on Nauru or Papua New Guinea.