The mother of an Iranian asylum seeker who took his own life on Nauru last month has been told his body will be sent to Iran against her wishes and not to Australia where he had relatives.

Fazileh Mansour Beigi has told Guardian Australia she will stand vigil by the container where Fariborz Karami’s body is being held inside the Australian-run regional processing centre.

She vowed to “stand by this container until the Australian government arrives to throw my kids and my body into the sea and release all of us together”.

She has told Australian Border Force that she wishes for her eldest son to be buried in Australia, where her sister can perform a burial service, and visit his grave.

And she has written – through the camp management company Canstruct and the ABF – to the home affairs minister, Peter Dutton, pleading for compassion.

In it she says:

I thought for a mother that nothing would be as torturous as seeing her children suffer for five years, in the heat above 40 degrees, in a tent filled with mould, dirt and a rusted fan. But, now, as a mother, more painful for me is seeing my son’s dead body, disrespected in front of my eyes, in a fridge inside a rusty container for 19 days … My only wish is that you bury Fariborz’s body with respect in Australia, as soon as possible, so he can rest in eternal peace and my 12-year-old son won’t have the same destiny as his older brother.

The Guardian has seen letters written to camp managers by Fariborz’s younger brother but has chosen not to reproduce or quote from them because of their graphic nature. Doctors on the island have, over years, consistently raised concerns about the boy’s mental health.

In English, Fazileh wrote to the ABF formally requesting that her son’s body be buried where her sister could oversee the funeral service.

Fazileh, the mother of Fariborz Karami, outside the connex container where his body is being held. Photograph: Supplied

The 26-year-old died on 15 June. An autopsy revealed a drug overdose. An embalmer has been on the island for several days, Guardian Australia has been told, to prepare the body for burial.

Karami’s mother, wife, brother and close friends have been able to see his body over the past 24 hours. ABF officers insisted the body could only be buried in Nauru – for which the family would have to pay – or Iran.

Mansour Beigi has rejected both options as they would mean her family is permanently separated from her son’s resting place. She says she cannot return to Iran for fear of persecution, and Nauru has consistently refused to permanently resettle any refugees or asylum seekers.

Karami does have other relatives in Iran, from which, it is understood, he was estranged.

Mansour Beigi said the delay in burying her son was culturally insensitive and had caused her further grief.

“Many times, I have requested ABF to allow me to bury my child while his body and face are still looking beautiful,” she told Guardian Australia. “Now he is bruised, swollen and not even buried. I will stand by this container until the Australian government arrives to throw my kids and my body into the sea and release all of us together.”

In response to questions about Fariborz Karami, the home affairs department has maintained: “Further inquiries should be referred to Nauruan authorities … The department does not comment on individual cases.”

Sources on Nauru have told Guardian Australian that ABF has ultimate authority over the regional processing centre, and has had “complete control” over Karami’s body and the movement of his family on the island.

Questions to the Nauruan government have not been answered.