U.S. troops who risked their lives battling insurgents in Afghanistan for years faced another unseen enemy at bases across the country: toxic fumes from open-air “burn pits” that were used long after orders to phase them out.

A scathing new watchdog report details the extent of the “indefensible” practice. The report says the U.S. military even spent millions on incinerators as a trash-burning alternative, yet several of them did not work or were never turned on, wasting $20 million and leaving troops to breathe pollution from the burn pits instead.

“Given the fact that [the Department of Defense] has been aware for many years of the significant health risks associated with open-air burn pits, it is indefensible that U.S. military personnel, who are already at risk of serious injury and death when fighting the enemy, were put at further risk from the potentially harmful emissions from the use of open-air burn pits,” John Sopko, special inspector general for Afghanistan reconstruction (SIGAR), said in his findings.

The report coincides with a separate class-action lawsuit -- by hundreds of veterans who say they were sickened by exposure to burn pits over the course of the 13-year war -- that is moving closer to a trial.

That suit, lodged against private contractors KBR and its former parent company Halliburton Co., is headed back to U.S. District Court in Maryland after the U.S. Supreme Court declined in January to take up the defendants’ appeal for a dismissal.

KBR, which had a government contract to provide waste management on overseas bases, says it was not operating in Afghanistan during the period covered in the SIGAR investigation, and the company is not mentioned in the report. But the report’s strong words about the health hazards could bolster the arguments made by servicemembers who say their proximity to the burn pits resulted in protracted medical issues -- including asthma, acute respiratory illness, immunity disorders and even cancer.

“[The] IG report confirms what we have alleged in the lawsuit,” said Susan Burke, the lead attorney representing the veterans.

'No U.S. installation in Afghanistan has ever been in compliance'

— SIGAR report

Neither the government nor KBR has acknowledged a link between the burn-pit emissions and long-term illness. DOD and Veterans Affairs (VA) officials contend that studies are ongoing. The VA recently opened an Airborne Hazards and Open Pit Registry for veterans to share their symptoms and experiences in hopes of learning more. However, on its website the VA states, “at this time, research does not show evidence of long-term health problems.”

According to Sopko, the pits burned a peak of 410 tons of solid waste a day throughout the war -- everything from vehicle parts, plastics and medical waste to prohibited hazardous materials like batteries and tires. The toxic brew created a black plume of smoke that lingered in the air over barracks and other common areas, soldiers have long reported. After returning veterans came to members of Congress complaining of health issues, new guidelines were passed in 2009 to phase out the burn pits on installations with more than 100 people starting in 2011. Incinerators were supposed to be moved in as a clean alternative.

It didn’t work out that way. According to the report, “[U.S. Central Command] officials told us that no U.S. installation in Afghanistan has ever been in compliance,” and SIGAR investigators personally witnessed the continued use of pits in the field through 2013.

The investigation was conducted from October 2012 to June 2014, at four out of the nine U.S. bases where incinerators were reportedly installed after 2010. SIGAR found that out of the $81.9 million taxpayers spent on 23 incinerators, $20.1 million was wasted because at least eight of them on those four sites were never used.

The report blames contractors hired by the military for faulty or incomplete installation of the incinerators. It also points a finger at DOD for paying contractors despite the fact incinerators were never turned on, or were left with deficiencies that prevented them from fully operating.

SIGAR found that at places like Camp Leatherneck, which housed an average of 20,000 people at any one time, “open-air burn pit operations continued on the base until October 2013, an additional 16 months” after four incinerators had been installed there.

Meanwhile, hundreds of veterans have joined the class-action lawsuit against KBR and Halliburton. Since 2001, KBR has held a stake in the multi-billion dollar contract with the Pentagon in both Afghanistan and Iraq to provide, in part, waste management and other services on U.S. bases.

The Texas-based company argues the suit should be dismissed because since the company was working on behalf of the U.S. military, which cannot be sued in U.S. district courts for command decisions in wartime, KBR is also exempt from such litigation.

Reached by FoxNews.com, officials at KBR stressed the company was out of Afghanistan by September 2010 and not connected to the specific base issues covered in the SIGAR report. “The report highlights the fact that the military had to make difficult decisions on waste disposal based on the complex war-time operational environment at the time,” KBR spokesman Richard Goins said.

Nevertheless, Burke claimed the report backed up lawsuit claims that the potentially harmful effects of burn-pit emissions were known long before attempts to shut them down. The class-action suit alleges KBR did not take proper precautions to mitigate the effects of the burn-pit emissions and in some cases, exacerbated them. KBR denies those charges, saying it consistently followed military guidelines.

“The health hazards, sadly, are all too clear to me,” said Burke, referring to her clients, a number of whom have been diagnosed with a rare condition called constrictive bronchiolitis, an irreversible scarring and inflammation of the lung’s smallest passageways that can occur with exposure to toxins.

Kelly Kennedy, a spokeswoman for Bergmann & Moore, a veterans’ disability law firm and advocacy group, said the importance of Thursday’s report could not be underestimated.

“The thing that stuck out at me was that they were clearly worried about the health effects of those exposed,” she told FoxNews.com, noting the government’s hesitancy to “own” the problem. “Now you have this report just slamming the military’s oversight on this. Across the board, they were handling it badly.”

KBR officials stressed there is no proven link between the burn pits and veterans’ illnesses, and accused opponents in the lawsuit of “overstating their case and misstating the facts.”

Calls to the Pentagon press office were not returned on Thursday. In a written response, Maj. General John Murray, deputy commander of support for U.S. forces in Afghanistan, told the SIGAR, “the safety, of our Soldiers, Sailors, Airmen, Marines and Civilians is always our top priority.”

“Although this report clearly identifies areas for improvement, it does not fully account for the difficult and complex operational environment that led commanders to make some very difficult decisions.”