GENEVA (Reuters) - A U.N. human rights panel urged the United States on Monday to end widespread detention of would-be immigrants including asylum-seekers, saying the practice has “grown exponentially” and violates international law.

U.S. Immigration and Customs Enforcement (ICE) agents arrest an immigrant in San Clemente, California, May 11, 2017. REUTERS/Lucy Nicholson

The holding of migrants and would-be refugees in custody is often “punitive, unreasonably long, unnecessary and costly” and should be used only as a last resort, the panel said in a 23-page report to the United Nations Human Rights Council.

Each year, an estimated 352,850 people are detained across the United States pending the outcome of their immigration proceedings at a cost of about $2 billion, it said.

The independent experts, who form the U.N. working group on arbitrary detention, were reporting on their mission last October at the invitation of the Obama administration.

“The Working Group is of the view that all administrative detention, in particular of immigrants in an irregular situation, should be in accordance with international human rights law; and that such detention is to be a measure of last resort, necessary and proportionate and be not punitive in nature, and that alternatives to detention are to be sought whenever possible,” the report said.

In Washington, a White House spokeswoman said “That’s a question for the U.N.” when asked to comment on the panel’s findings. The Department of Homeland Security did not immediately respond to a request for comment.

President Donald Trump has backed legislation to crack down on illegal immigrants, describing it as vital to protect American lives.

The U.N. experts interviewed 280 detainees during their visits to nine prisons in Texas, California and Illinois. Some were remote locations with limited access to legal services.

They reported seeing immigrants and asylum seekers held in “punitive conditions” comparable to those of convicted criminals despite their right to seek asylum under international law.

In some cases, the length of detention pending immigration proceedings was “unreasonable”, lasting from six months to more than one year without resolution.

The experts voiced concern at Trump’s executive order in January and an implementing memorandum that “lay the groundwork for expanding the existing detention system by increasing the number of individuals subject to immigration detention”.

“Under the order, apprehended individuals may be detained ‘on suspicion’ of violating federal or state law, which includes unauthorized entry,” they said.

They received information in March that the Department of Homeland Security was considering separating children from parents caught crossing the border, “in an attempt to deter illegal immigration from Mexico,” they said.

“This is particularly serious given the increasing trend of unaccompanied children migrating to escape violence and reunite with family members.”