(CNN) Airstrikes by the US-led coalition in Raqqa, Syria, probably breached international humanitarian law and potentially amount to war crimes, according to a report by Amnesty International that is being hotly contested by the Pentagon.

The rights group accuses the coalition of killing and injuring thousands of civilians in attacks that were at times "disproportionate or indiscriminate," during its offensive to flush ISIS militants from their de facto capital.

"The coalition's claims that its precision air campaign allowed it to bomb (ISIS) out of Raqqa while causing very few civilian casualties do not stand up to scrutiny," said Amnesty's senior crisis response adviser, Donatella Rovera.

"On the ground in Raqqa we witnessed a level of destruction comparable to anything we've seen in decades of covering the impact of wars."

A spokesman for the US-led coalition fighting ISIS slammed the report, saying that Amnesty never approached the Pentagon about its findings and was out of line for suggesting the coalition has violated international law.

"They are literally judging us guilty until proven innocent, that's a bold rhetorical move by an organization that fails to check the public record or consult the accused," Col. Thomas Veale told reporters at the Pentagon via a video briefing from Baghdad.

"They never asked us for a comment, an interview, or a courtesy check of the draft, they also failed to check the public record thoroughly," Veale said.

He also criticized Amnesty for recommending that the coalition develop a process for canceling a strike if it's deemed indiscriminate or disproportionate, saying that the coalition already had this process in place.

Veale added that the number of civilian casualties can't ever be really known.

"As far as how do we know how many civilians were killed -- I'm just being honest, no one will ever know," Veale said. "Anyone who claims they will know is lying."

The report, "War of Annihilation," details the loss of civilian life in Raqqa, based on interviews with 112 civilians at the sites of 42 coalition airstrikes.

It illustrates the cases of four civilian families who, between them, lost 90 relatives and neighbors, including 39 from one family alone. Almost all were killed by coalition airstrikes, the report alleges.

"They are part of a wider pattern and provide a strong prima facie case that many coalition attacks that killed and injured civilians and destroyed homes and infrastructure violated international humanitarian law," the report states.

The report illustrates the difficult choice many civilians faced of either choosing to flee and be killed by ISIS snipers or risk being hit in coalition strikes.

The US-led coalition says it does everything it can to minimize harm to civlians in its airstrikes.

The Badran family, which lost 39 members and 10 neighbors in four separate coalition strikes, fled from place to place as front lines in the city rapidly shifted.

Rasha Badran, one of the survivors, told Amnesty that she thought the coalition forces would target only ISIS militants.

"We were naive. By the time we realized how dangerous it had become everywhere, it was too late; we were trapped," she told Amnesty.

Veale said that the coalition was willing to work with anyone to assess allegations of civilian casualties.

"I can tell you with confidence we are always willing to re-evaluate cases based on new or compelling evidence," Veale said, adding that, "as I speak people are looking at that article and trying to correlate those claims to the strike log and how the battle of Raqqa unfolded as our participation went in it."

"We are open to working with anyone," Veale said. "We are just as willing to work with Amnesty International, as I said I wish we had worked with them earlier but they didn't come to us, they just went ahead and published"

Coalition 'leveled' Raqqa

The coalition's offensive in Raqqa began a year ago, with US, British and French forces taking part.

Tens of thousands of airstrikes were carried out in the city, Amnesty said, adding that US forces "admitted to firing 30,000 artillery rounds during the offensive." It said US forces were responsible for 90% of coalition strikes.

Amnesty said that ISIS' four-year rule in Raqqa was "rife with war crimes," but that did not relieve the coalition of its obligation to minimize harm against civilians.

"What leveled the city and killed and injured so many civilians was the US-led coalition's repeated use of explosive weapons in populated areas where they knew civilians were trapped. Even precision weapons are only as precise as their choice of targets," Rovera said.

Responding to the report, the US-led mission to Syria said it made rigorous efforts to avoid civilian casualties.

"The coalition applies rigorous standards to our targeting process and takes extraordinary efforts to protect non-combatants," it said in a statement sent to CNN.

It added that it had been transparent about its strikes and routinely assessed any allegations of civilian casualties. It is committed to transparency "when civilian casualties unintentionally occur," the statement said.

Amnesty International is accusing the US-led coalition of flattening the city of Raqqa.

A British Ministry of Defense spokesperson said its mission in Syria fully complied with international humanitarian law, also adding it had been open and transparent throughout the offensive and detailed each of its nearly 1,700 strikes.

"We do everything we can to minimize the risk to civilian life through our rigorous targeting processes and the professionalism of the (Royal Air Force) crews but, given the ruthless and inhuman behavior of (ISIS), and the congested, complex urban environment in which we operate, we must accept that the risk of inadvertent civilian casualties is ever present," the spokesperson said in statement.