Two leading humans rights groups are sharply criticizing the Pentagon for saying this week that the U.S. military will never know the exact number of civilians it has killed in the four-year fight against the Islamic State.

Amnesty International USA and Human Rights Watch blasted the admission as an abdication of responsibility to avoid civilian casualties and thoroughly investigate the collateral damage of war.

"When the U.S. tells the world it doesn't care enough to track the deaths of civilians it is causing, that's a green light for belligerents around the world to take the exact same attitude," said Sarah Leah Whitson, the Middle East and North Africa director for Human Rights Watch.

U.S. Army Col. Thomas Veale, a spokesman for the U.S.-led coalition battling ISIS militants, made the comments on Tuesday in response to a reporter's question following the release of an Amnesty International report that claims U.S. forces killed thousands of civilians last year in the battle to retake the Syrian city of Raqqa.

"As far as how do we know how many civilians were killed — I'm just being honest — no one will ever know," Veale told reporters via teleconference from Baghdad. "Anyone who claims they will know is lying, and there's no possible way."

Veale said that civilian casualties as a result of airstrikes by the U.S. and its allies are "extremely unfortunate" and "a terrible, awful part of this war" against ISIS-linked fighters who swept across Iraq and Syria in 2014, terrorizing civilians and destroying ancient ruins.

Veale added that the U.S. military was doing its best to assess the full toll of its actions in the battle against the Islamic State, which began in 2014: "We're trying to evaluate the evidence that we have, the best evidence we have."

The Amnesty International report in question, titled "War of Annihilation," accused the U.S. military of being responsible for thousands of civilian deaths during a four-month push to wrest control of the ISIS stronghold of Raqqa in 2017.

"There is strong evidence that Coalition air and artillery strikes killed and injured thousands of civilians, including in disproportionate or indiscriminate attacks that violated international humanitarian law and are potential war crimes," Amnesty said in a summary of the report published on its website.

The report also notes that ISIS-linked fighters used civilians as "human shields."

In an annual report that was released to the public on June 1, the Pentagon said U.S. military operations in Afghanistan, Iraq, Syria and Yemen killed "approximately" 499 civilians and injured another 169 in 2017, adding that its estimates were based on reports the Defense Department deemed "credible."

A director with Amnesty International USA said Veale's statements sounded like a "cop-out," as if the U.S. government were essentially saying: "We killed thousands of people, we don't know who they are, too bad."

"It's really the United States' responsibility to know the death and destruction it causes when it goes in and bombs" what it believes to be enemy combatants in a foreign country, said the Amnesty director, Daphne Eviatar.

"It's just unconscionable," Eviatar added.

The United States "can't know with 100 percent certainty" how many lives are claimed in the chaos of war, said Stephanie Savell, co-director of the Costs of War project at Brown University, an initiative that collects information on the consequences of the Afghanistan and Iraq wars.

And yet Veale, in his comments to reporters, "completely delegitimized the urgent need for the U.S. to be accountable for the civilian deaths it's causing abroad," Savell said.

In an email to NBC News, Veale said the U.S. military's "civilian casualty assessment team is poring through the Amnesty report for new or compelling evidence." He added that the team's findings would be included in the Defense Department's next monthly report of civilian casualties, known as CIVCAS.

People inspecting damage from airstrikes and artillery shelling in the northern Syrian city of Raqqa. Aamaq News Agency via AP

Amnesty's report grew out of a visit by researchers to more than 40 strike locations and interviews with more than 100 witnesses and survivors of the airstrikes and artillery barrages, according to Eviatar, the Amnesty director.

Amnesty documented the experiences of four families in Raqqa, featuring first-person accounts of the devastation. Rasha Badran, one of the airstrike survivors, told the group she lost 39 family members — including her baby girl — in four different strikes the report claims were carried out by U.S.-led forces.

"It was completely silent except for the planes circling above," Badran is quoted as saying. "We hid in the rubble until the morning because the planes were circling overhead. In the morning, we found Tulip's body; our baby was dead. We buried her near there, by a tree."

At the Pentagon briefing on Tuesday, Veale took issue with the Amnesty report, saying its authors never reached out to the military for comment and "failed to check the public record."

An 18-month investigation by The New York Times, published last November, found that the U.S.-led air campaign against ISIS has been "significantly less precise than the coalition claims."

Secretary of Defense Jim Mattis told reporters in March 2017 that "there is no military force in the world that has proven more sensitive to civilian casualties" than that of the United States.