20 Iraqi Christians have been held in a Californian detention centre for more than months.

The Chaldean Christians are confined at Otay Detention Centre in San Diego after fleeing persecution at the hands of ISIS. They are being held by immigration officials after trying to cross the Mexico-USA border without proper documentation.

The detainees have family members to sponsor their application which typically allows asylum seekers to be released while their cases are processed. Despite this, the US Immigration and Customs Enforcement (ICE) has offered little explanation for their prolonged detention.

San Diego's Chaldean community held a prayer service outside the Otay Detention Centre last month in protest. The refugees survived Islamic State only to land in prison elsewhere, attenders of the service said.

Waheed Butrus of El Cajon, just outside San Diego, said he attended the event to call for the release of his son-in-law and granddaughter.

Butrus, 61, said he's unsure of why they've been held at the prison for so long. He visits them at the weekends.

"I'm very sad. I think about them every day," he said. "It's an injustice."

The protest "was our way of telling our Christian brothers and sisters that they will not be forgotten," Mark Arabo, a spokesperson for the San Diego Chaldean community said.

"It is important to understand that the State Department ended all processing for religious minorities out of Iraq, so there is no legal way for an individual to come from Iraq to the US," Arabo explained.

"The biggest obstacle these families face is not ISIS now though. It is our America [Immigration and Customs Enforcement]."

Lauren Mack, spokeswoman for ICE, confirmed that there are 27 Iraqi nationals in ICE custody at the facility but said she couldn't comment on individual immigration cases without the detainees' written consent.

The plight of the Chaldeans follows a similar pattern to thousands of other religious minorities fleeing the persecution of the Middle East.

More than 100,000 Christians have fled Iraq since the rise of ISIS, tens of thousands of whom have found permanent homes in El Cajon.