The Syrian civil war’s impact on the health of Syria’s children is far more insidious than has been widely understood, a leading children’s advocacy group reported Sunday, with large numbers dying or at risk from chronic and preventable diseases that have flourished because the country’s public health system has basically collapsed.

In a report timed to coincide with the start of the fourth year of the conflict, the group, Save the Children, said the effects of untreated illnesses on Syrian children were only partly reflected in the documented statistics. They show that at least 1.2 million children have fled to neighboring countries, that 4.3 million in Syria need humanitarian assistance and that more than 10,000 have died in the violence.

“It is not just the bullets and the shells that are killing and maiming children,” said the report, “A Devastating Toll.” The conflict, which began in March 2011, has left a “shattered health system resulting in brutal medical practices that have left millions of children suffering,” the report said.

The report asserted that “several thousands of children” had died because of greatly reduced access to treatment for diseases including cancer, epilepsy, asthma, diabetes, hypertension and kidney failure.