Human rights groups said Tuesday that airstrikes in and around the ISIS-controlled city of Manbij had killed many residents and wounded dozens more.
Carter said the coalition was aware of the reports.
"We'll ... continue to do all we can to protect civilians from harm," he said at anti-ISIS coalition defense ministerial news conference at Joint Base Andrews near Washington. "Being scrupulously careful in avoiding civilian casualties and being transparent about this issue is a reflection of the civilized nature of this coalition."
The Manbij area is the last large tract of land along Syria's northern border with Turkey under ISIS
control, according to the UK-based Syrian Observatory for Human Rights. Considered a strategic supply point between Raqqa and Turkey, it has become the site of intense fighting since a U.S.-backed alliance of Kurdish and Arab militias entered the city in June.
Though accounts vary, human rights group said airstrikes this week killed dozens of people, pushing the death toll past 100.
The Syrian Observatory for Human Rights
said airstrikes Tuesday killed 56 people, including 11 children, in the countryside north of Manbij.
Citing accounts from local activists and documentary evidence, Amnesty International
said airstrikes on nearby al-Tukhar village Monday and Tuesday killed at least 60 men, women and children in their homes.
The National Coalition of Syrian Revolutionary and Opposition Forces
said strikes in Manbij and al-Tukhar killed 95 people, and a number of civilians remained trapped under building rubble.
The groups condemned the attacks and called on coalition forces to step up efforts to prevent civilian deaths.
"The bombing of al-Tukhar may have resulted in the largest loss of civilian life by coalition operations in Syria. There must be a prompt, independent and transparent investigation to determine what happened, who was responsible, and how to avoid further needless loss of civilian life," said Magdalena Mughrabi, interim deputy director of the Middle East and North Africa Program at Amnesty International.
"Anyone responsible for violations of international humanitarian law must be brought to justice and victims and their families should receive full reparation."
Col. Chris Garver, a spokesman for the U.S. military program charged with fighting ISIS, confirmed that airstrikes were conducted Monday near Manbij.
The U.S.-backed alliance of Kurdish and Arab militias known as the Syrian Democratic Forces launched the offensive in June. The operation aims to keep ISIS fighters from crossing into Turkey and ensure that foreign fighters cannot enter Syria.
A senior administration official called it a hub of ISIS external operations and a "critical supply node" on the road to Raqqa, an ISIS stronghold.
"Cut them off there and they're totally isolated in Raqqa. So it's critical, strategic, and we have now launched an operation long in planning to go after it," the official told CNN in June
The Syrian Observatory for Human Rights said Manbij's population density, which is estimated to be in the tens of thousands, makes it difficult to avoid civilian deaths.