As Defense Secretary Ash Carter announces that the U.S.-led military coalition has "already begun laying the groundwork" for an attack on the Syrian city of Raqqa, Amnesty International has warned that nearly a dozen attacks carried out by the forces may have killed as many as 300 civilians in the country in possible violation of international humanitarian law.

The coalition may be "significantly underestimating" damage it's caused to civilians and civilian infrastructure, Amnesty said in a press statement released Wednesday.

The human rights organization says the Defense Department has yet to respond to a memo it sent sent nearly one month ago that compiled information—based on local rights groups, eyewitnesses, and media accounts—about the attacks, and questioned whether coalition forces took the necessary steps to prevent civilian casualties.

Among those attacks was an August 11, 2015 strike targeting a building used by an armed group to produce mortars. A coalition airstrike hit that building, and satellite images obtained by the group show that two nearby homes were destroyed as well, killing eight civilians including six children. It is unclear whether any combatants were killed in the strike.

Talha al-Amouri was an eyewitness to that attack. His sister-in-law was mother to five of the children killed. She also suffered a stillbirth following the attack.

"How could they have known that there was an ammunitions factory but not that there were homes with civilians nearby?" he said to Amnesty.

Acording to Lynn Maalouf, deputy director for research at Amnesty International's Beirut regional office, "U.S. authorities should have taken steps to minimize that risk, including by issuing a warning, if feasible, or delaying the attack until civilians could be adequately protected, or canceling it if it was likely to be disproportionate."

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In that event and the others analyzed by Amnesty, "Coalition forces failed to take adequate precautions to minimize harm to civilians and damage to civilian objects. Some of these attacks may constitute disproportionate or otherwise indiscriminate attacks," Maalouf said.

She added, "It's high time the U.S. authorities came clean about the full extent of the civilian damage caused by coalition attacks in Syria. Independent and impartial investigations must be carried out into any potential violations of international humanitarian law and the findings should be made public."

Amnesty also noted that research by other leading human rights organizations and monitoring groups has estimated that the total number of civilians killed in Syria by coalition forces may top 1,000, a stark contrast to statement last year by a U.S. military official that "Our coalition airstrikes are the most precise and disciplined in the history of aerial warfare."

Meanwhile, the U.S air war on Syria continues, and Iraqi and Kurdish forces, backed by U.S. airstrikes and special operations, are continuing an offensive to take back the Iraqi city of Mosul from Islamic State (ISIS or ISIL) fighters, which human rights campaigners warned could worsen the humanitarian situation and lead to "bloodshed of civilians on a massive scale."

The "same sense of urgency and focus" used in the Mosul operation, Carter said Tuesday, will transfer over to an assault on the Islamic State's 'stronghold' in Syria.

Speaking alongside his French counterpart, Defense Minister Jean-Yves Le Drian, Carter said, "we've already begun laying the groundwork with our partners to commence the isolation of Raqqa," adding, "This is one of our campaign plan's core objectives; destroying ISIL's parent tumor in Iraq and Syria."

Transparency project Airwars estimates that over the last two years, coalition actions have killed over 1,700 civilians—and possibly over 2,500— in Iraq and Syria.