Britain-based Syrian Observatory for Human Rights said at least 56 civilians were killed in air strikes north of Manbij
The head of the opposition Syrian National Coalition (SNC) called for a suspension of the US-led air campaign against Islamic State 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 Islamic State-held city.
SNC president Anas al-Abdah said the strikes should be halted while the incidents were investigated, according to a statement issued late on Wednesday, and warned that the killing of civilians by the US-led air campaign would "prove to be a recruitment tool for terrorist organisations".
"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.
US defence secretary Ash Carter said on Wednesday the US-led force would look into the reports of civilian casualties around Manbij.
The Observatory said the dead from Tuesday's strike included 11 children. Pictures on social media purporting to be from the scene showed dust-covered corpses of two young children next to rubble.
Syria's foreign ministry said Tuesday's air strike, which hit the village of Toukhan north of Manbij, was carried out by French forces, while Monday's strike was by US jets.
"(Syria) condemns, with the strongest terms, the two bloody massacres perpetrated by the French and US warplanes and those affiliated to the so-called international coalition which send their missiles and bombs to the civilians instead of directing them to the terrorist gangs," it said in a letter sent to the United Nations this week, according to state news agency SANA.
A spokesman for the US-led alliance said there were "multiple national aircraft providing strikes in Manbij. So how the Syrian government knows who conducted what strike, I question."
The Western-backed Free Syrian Army, an umbrella grouping of factions which has fought against both President Bashar al-Assad and against Islamic State militants, also condemned what it called the "shocking massacres" near Manbij.
"We will not allow any crime to be justified under the pretext of combating terrorism," it said in a statement signed by more than 30 armed factions.