New evidence has emerged of dangerously subpar medical care in US immigration detention centres that has caused preventable deaths, Human Rights Watch (HRW) said in a report published on Monday.

The New York-based rights group highlighted "systemic failures" that have led to at least 18 preventable deaths between 2012-15, a figure that could be significantly larger.

"The data reveals that people in immigration detention died needlessly under the Obama administration, even with its attempts at reform," said Grace Meng, senior US researcher at HRW, according to the group's website.

Some of the issues identified include "unreasonable delays in care and unqualified medical staff".

HRW said the situation could worsen under President Donald Trump's ramped-up deportation and detention plans.

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The report is based on independent analyses by medical experts of records from US Immigration and Customs Enforcement's (ICE) own investigations into 18 deaths in detention from 2012 to 2015, and medical records of 12 additional people from 10 privately and publicly operated facilities across the country.

Human Rights Watch also interviewed more than 90 people who are or were detained, as well as family members, lawyers, immigration advocates, and correctional health experts.

The medical experts found numerous incidents of substandard and dangerous medical care, including failure to follow up on symptoms that required attention.

They also uncovered medical personnel practising beyond the scope of their licences and expertise, severely inadequate mental healthcare, the misuse of solitary confinement for people with mental health conditions, and sluggish emergency responses.

HRW presented the case of Raul Ernesto Morales-Ramos, who died of organ failure caused by cancer in 2015, after being detained at the Adelanto Detention Facility in California.

Two experts who reviewed records of his death found there had been symptoms of widespread cancer two years earlier, but his condition went unaddressed until a month before he died. Throughout this time, Morales-Ramos is said to have repeatedly begged for care.

"The sheer number and consistency of cases involving inadequate medical care point to a crisis that warrants immediate action," said Christina Fialho, a lawyer and co-executive director of the Community Initiatives for Visiting Immigrants in Confinement (CIVIC), a group that contributed to the report.

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US Immigration and Customs Enforcement said it would look into the allegations and defended its healthcare provided to detainees in a statement emailed to Al Jazeera.

"US Immigration and Customs Enforcement will review the report to determine what changes, if any, should be made based on its recommendations," it said.

"ICE is committed to ensuring the welfare of all those in the agency's custody, including providing access to necessary and appropriate medical care. Comprehensive medical care is provided to all individuals in ICE custody."

The US currently detains about 40,000 people a day, or more than 400,000 a year, at an annual cost of US$2bn. Many people in detention are blocked under US law from having a bond hearing to determine whether their detention is necessary.

The Trump administration's recent request for supplemental funding included a request for $1.2bn for increased detention capacity from the current 34,000 beds to an unprecedented 45,700.