The head of the opposition Syrian National Coalition called for a suspension of the U.S.-led air campaign against ISIS in Syria while reports of dozens of civilian deaths from air strikes around the northern city of Manbij are investigated.

The Britain-based Syrian Observatory for Human Rights said at least 56 civilians were killed in air strikes north of Manbij on Tuesday, a day after it said 21 civilians were killed in a northern district of the besieged, ISIS-held city.

SNC president Anas al-Abdah said the strikes should be halted while the incidents were investigated, and warned that the killing of civilians by U.S.-led aircraft would “prove to be a recruitment tool for terrorist organizations.”

“It is essential that such investigation not only result in revised rules of procedure for future operations, but also inform accountability for those responsible for such major violations,” Abdah wrote in a letter to foreign ministers of countries in the anti-Islamic State alliance.

U.S. Defense Secretary Ash Carter said on Wednesday the U.S.-led force would look into the reports of civilian casualties around Manbij.

The Observatory said the dead from Tuesday’s air strike included 11 children. The United Nations children’s agency UNICEF said it had been told that families were preparing to flee when the villages they were in came under air attack.

“UNICEF estimates that 35,000 children are trapped in and around Manbij with nowhere safe to go,” the agency’s representative in Syria, Hanaa Singer, said.