More than 11,000 children have died in Syria's civil war, including 128 killed by chemical weapons and hundreds targeted by snipers, a British think-tank says.

The Oxford Research Group, which specialises in global security, said in a new study that there were 11,420 recorded deaths of children aged 17 years and under.

The report, entitled Stolen Futures: The Hidden Toll of Child Casualties in Syria, analyses data from the beginning of the conflict in March, 2011, until August, 2013.

The think-tank said that, of the 10,586 children whose cause of death was recorded, 128 were killed by chemical weapons in Ghouta, near Damascus, on August 21, 2013.

The United States and other world powers blamed that attack on President Bashar al-Assad's regime.

The regime has since agreed to destroy its entire chemical weapons arsenal as part of a deal to head off US military strikes.

The think-tank said 764 children were summarily executed and 389 were killed by sniper fire in the conflict.

Explosive weapons have caused more than 70 per cent of the child deaths, while small arms fire accounts for more than a quarter, according to the study.

Cruel child killings highlight need for peaceful end to war

"What is most disturbing about the findings of this report is not only the sheer numbers of children killed in this conflict, but the way they are being killed," report co-author Hamit Dardagan said.

"Bombed in their homes, in their communities, during day-to-day activities such as waiting in bread lines or attending school; shot by bullets in crossfire, targeted by snipers, summarily executed, even gassed and tortured.

"All conflict parties need to take responsibility for the protection of children and ultimately find a peaceful solution for the war itself."

Oxford Research Group added that the number of boys killed out-numbered girls killed by a ratio of about two to one.

Those children in older age groups were targeted more often than younger children. Boys aged between 13 and 17 were the most frequent victims of targeted killings.

"The data we analysed indicates that bombs and bullets alone ended the lives of 10,000 Syrian children in 30 months of war," Mr Dardagan said.

"The world needs to take a much closer interest in the effects of the conflict on Syria's children."

AFP