(CNN) Six US senators are asking the Department of Homeland Security to take immediate steps to ensure private-prison operators aren't using forced labor at immigrant detention centers, after allegations that detainees feel coerced to work for $1 per day or less to get sufficient food and basic services.

Citing reporting last month by CNN , the senators -- five Democrats and one Independent -- sent a letter to DHS Secretary Kirstjen Nielsen raising concerns about privately run detention centers' operation of what is supposed to be a voluntary federal work program for detainees. They expressed concern about the facilities "withholding adequate food, hygienic supplies, and contact with loved ones from immigrant detainees. When detainees are unable to receive outside funds to purchase supplies, they feel coerced into volunteering to participate in work programs. This includes performing tasks such as mopping, sweeping, waxing floors, scrubbing toilets and showers, washing dishes, doing laundry, cleaning medical facilities, cutting hair, and preparing meals."

The August 13 letter also cited a lawsuit alleging that detainees who objected to volunteering "subsequently faced punishments such as segregation, solitary confinement, threat of physical harm, and referral for criminal prosecution."

The letter was signed by senators Kirsten Gillibrand of New York, Elizabeth Warren of Massachusetts, Richard Blumenthal of Connecticut, Bernie Sanders of Vermont, Mazie Hirono of Hawaii, and Cory Booker of New Jersey.

"American taxpayers should not be subsidizing the abuse and exploitation of immigrants in private detention centers. The Department of Homeland Security has a legal and moral obligation to take immediate action to ensure that private detention centers follow the law and treat all people in their custody with dignity and respect," Warren said in an email.

Gillibrand called the alleged coerced work by detained immigrants "illegal" and "inhumane." "Secretary Nielsen should end these practices immediately and put in place much-needed accountability and oversight measures to ensure that this never happens again," she told CNN in an email.

DHS did not immediately respond to requests for comment on the letter.

As CNN reported, detention centers operated under contract by The GEO Group and other private-prison operators save millions of dollars yearly by paying detainees a dollar or less a day for such work instead of hiring employees at standard wages. For-profit prison companies have thrived under the Trump Administration, which rescinded a memo phasing out the use of private prisons and awarded the companies tens of millions of dollars in new contracts to house detained immigrants, among others.

The GEO Group, when contacted, said, "While our company has strongly, consistently refuted these baseless allegations, we have also always welcomed oversight, transparency, and accountability."

"All ICE Processing Centers managed by our company comply with the 2011 performance-based national detention standards, including those which govern the federal government's Voluntary Work Program," spokesperson Pablo Paez said in a statement. "Our facilities are subject to full-time, on-site government monitoring and routine and unannounced reviews and audits. We welcome the continued oversight from the executive and legislative branches of the federal government and from independent accreditation agencies. We are confident that any thorough review of the facilities we manage on behalf of the federal government will validate the high-quality of the services our employees provide on a daily basis."

CoreCivic, a rival private-prison group that is the subject of the lawsuit mentioned in the senators' letter, called the claims "false and misleading." "All work at our ICE-contracted facilities are completely voluntary and operated in full compliance with ICE standards, including federally established minimum wage rates for detainee labor," said spokesperson Amanda Gilchrist.

She added, "While we know (immigration) is a highly charged, emotional issue for many people, much of the information being shared about our facilities is wrong and politically motivated."

At the time of CNN's investigation, ICE said that it "takes very seriously the health, safety and welfare of those in our care. The agency is committed to ensuring that those in our custody reside in safe, secure, and humane environments and under appropriate conditions of confinement."

At least five immigration detention centers operated by The GEO Group or CoreCivic face federal lawsuits over their detainee work programs. Last week, a US District Court judge in Washington granted class certification to current and former immigrant detainees working at The GEO Group's Northwest Detention Center in Tacoma, in a suit filed by Washington Attorney General Bob Ferguson. That suit charges the company with violating state minimum wage laws by paying detainees a dollar a day or less to work at the center. Another federal district court in Colorado granted class certification in March 2017 to migrants at a detention center in Aurora, Colorado, also run by The GEO Group.

Though ICE is not a defendant in those suits, ICE spokeswoman Carissa Cutrell said, in an email, that the agency doesn't comment on pending litigation.

Referring to the Washington lawsuit, GEO Group spokesman Paez said that although "the court ruled on this motion without prejudice, it has not ruled on the substance of the claims. As we have consistently stated, we intend to defend our company against these claims."

In the last few years, inmates at the centers have staged hunger strikes -- more than a dozen at the Northwest Detention Center alone -- to protest what they call inadequate food and poor conditions. Those strikes have been renewed in recent days, advocacy groups say. Northwest detainee Jose Roberto Garcia Garfias ended a hunger strike last week after five days, according to NWDC Resistance, an advocacy group that has mounted protests and supported repeated hunger strikes at that center. Cutrell said that ICE "could not provide further information on the case" of Garcia Garfias.

In Texas, the immigration legal services group RAICES said that hundreds of immigrant fathers at the Karnes County Detention Center began a hunger strike August 2. ICE officials denied that there was a hunger strike, characterizing the protest as a brief sit-in by fewer than 50 men.

The Northwest Detention Center sits in Rep. Adam Smith's district. Smith spoke with detainees on a visit to the center after a large hunger strike in 2014, and introduced a bill that session that aimed to improve living conditions and provide greater oversight of the centers. It failed to clear the House. Smith has introduced another bill this session that would require DHS to establish legally enforceable civil detention standards in line with those adopted by the American Bar Association.

In their letter to Secretary Nielsen, the senators said that "it is unacceptable that private companies are forcing labor upon detainees, in the name of the US government ... rather than hiring employees to operate their facilities. ... It appears private companies have been allowed to deprive individuals of basic needs in order to provide billion dollar operations with a ready supply of cheap labor to operate their facilities."

The letter also cited a June 2018 report from the DHS Inspector General describing a lack of proper oversight by ICE of detention centers, and a failure to ensure that centers comply with detention standards. The senators asked Nielsen to improve accountability and oversight at the centers, to close within two years any that don't comply with federal detention standards, and to provide enough funding for DHS's Office of Inspector General to continue and expand unannounced inspections at detention facilities.