Syria's opposition appealed Wednesday to the US-led coalition fighting the Islamic State group to halt its air strikes after dozens of civilians were killed in raids near an IS-held town.

In a letter to the alliance's foreign ministers, National Coalition president Anas al-Abdah demanded "an immediate suspension of the military operations of the international (anti-IS) coalition in Syria to allow for a thorough investigation of these incidents".

The Syrian Observatory for Human Rights on Tuesday said that children were among at least 56 civilians killed in strikes by the coalition as they fled Al-Tukhar in Aleppo province, near the key IS stronghold of Manbij.

"We believe that such incidents indicate a major loophole in the current operational rules followed by the international coalition in conducting strikes in populated areas," said the letter.

"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," it added.

Al-Abdah strongly condemned the "massacre" and held the international coalition totally responsible for what he called "the crimes that took place in Manbij".

"The Syrian people, as you are aware, have been murdered, maimed and tortured for over five years now at the hands of (President Bashar al-)Assad's murderous regime, Russia, Iran and allied militias in addition to ISIS and terrorist groups."

Al-Abdah warned the killing of Syrians now at the hands of the international coalition "will only push them further into a spiral of despair and, more importantly, will prove to be a recruitment tool for terrorist organisations."