As the war in Syria raged on at the end of 2016, nearly one in four civilians killed in aerial bomb attacks was a child, a new study published in The Lancet Global Health journal says.

More than 17,000 children have been directly killed by weapons — including guns, ground explosives and air bombs — throughout more than six years of war, the researchers say.

And that's likely a huge underestimation, lead author Debarati Guha-Sapir said, because the researchers were only able to get reliable data from areas not controlled by the Syrian government. That means children killed in government areas have not been included in the study's analysis.

In the non-government-controlled areas, the shift in strategy from ground battles to airstrikes in 2013 dramatically increased the number of child deaths while having relatively little effect on actual fighters, Guha-Sapir said.

By the end of 2016, she said, "bombs were killing five times more civilians than combatants."

For the first couple of years after conflict between rebel groups and President Bashar al-Assad's government erupted in 2011, guns were among the main weapons of war and shootings were the main cause of death for civilians. About nine per cent of those killed were children.

But the proportion of children killed skyrocketed as shelling and air bombs became the weapons of choice. In 2013, children made up 19 per cent of civilian casualties, according to the data used in the study. By the end of 2016, that percentage climbed to 23 per cent.

"The share of children who have faced violent deaths ... is very, very, very high," said Guha-Sapir. The professor of epidemiology at Université catholique de Louvain in Brussels has been estimating deaths from war for over a decade, including conflicts in Sudan, Congo-Kinshasa and Iraq.

Many children died in the other conflicts she has studied, Guha-Sapir said, but she believes they way they've died in Syria is different.

The majority of child casualties of war, in her experience, have died of "indirect" consequences after conflicts wipe out the infrastructure to deliver food, medicine and vaccinations, and leave children vulnerable to disease.

That's happened in the Syrian conflict too, Guha-Sapir said — but large numbers of children have been directly killed by bombs dropped into urban areas, like Aleppo and the suburbs of Damascus. According to the study data, airstrikes and air-dropped barrel bombs (large canisters filled with explosives and scrap metal) alone have killed more than 9,200 children.

A girl cries near a damaged car at a site hit by what activists said were barrel bombs dropped by government forces in Aleppo's Dahret Awwad neighbourhood January 29, 2014. (Saad AboBrahim/Reuters)

The numbers in the study give statistical backing to ongoing condemnation by groups like Amnesty International and Physicians for Human Rights of the use of barrel bombs.

The rights groups have long said that the weapons are an attack on civilians. According to Guha-Sapir's study, 97.1 per cent of those killed by barrel bombs were civilians. Only 2.8 per cent of deaths from barrel bombs were "opposition combatants." More than one quarter of those killed were children.

"Barrel bombs are inherently indiscriminate and despicable weapons," said Physicians for Human Rights spokesperson Susannah Sirkin in an email to CBC News. "The tally of death of civilians due to these grotesque and illegal weapons [in the study] testified to that."

Dr. Samantha Nutt, founder of War Child Canada, agreed that the study's numbers supported what NGOs and aid agencies are observing on the ground.

"We know aerial bombardment is extremely lethal, that it is very indiscriminate, that it was particularly devastating in Syria, that it was widely used, that buildings and hospitals were targeted, which would affect civilians disproportionately."

Neither Sirkin nor Nutt were involved in the study.

Even though the research didn't include information for all of Syria because it excluded government-controlled areas, the data appeared to be "quite legitimate," Nutt said.

She said that studies providing concrete numbers on "the human cost of war" are "absolutely critical."

"Embedded in those numbers you learn about ... how many families have lost children and lost loved ones and what kind of intergenerational trauma is going to be created coming out of that and what kind of resources and supports they're going to need moving forward," Nutt said.

The research was based on data collected by the Violation Documentation Center in Syria. The centre recorded 145,849 deaths (including both combatants and civilians) between March 18, 2011, and Dec. 31, 2016. The researchers excluded 2,219 of those death reports due to incomplete or irrelevant information and analyzed a total of 143,630 deaths.