Images of Syrian Children Capture Horror of 5-Year Civil War It's been five years since the war started.

 -- A week-long ceasefire is set to take effect in Syria at sundown on Monday. But there have been disturbing reports of new violence that has killed dozens, including children, hours before the fighting temporarily abates.

According to one Syrian human rights group, more than 2,000 people have been killed in fighting over the past 40 days in the city of Aleppo, including 700 civilians and 160 children.

On Saturday alone, presumed Russian or Syrian regime airstrikes on the rebel-held provinces of Idlib and Aleppo left more than 90 civilians dead, including 13 children in an attack on Idlib's city marketplace, the Britain-based Syrian Observatory for Human Rights said.

Since Syria plunged into civil war five years ago, dramatic images of children living in rebel-held areas and besieged cities have captured the horror of the country's crisis.

Last month, the haunting image of 5-year-old Omran Daqneesh caked with dust and blood stunned the world after the Syrian boy was pulled from the rubble of a bombed building in Aleppo. He was injured but alive. Images and video of Omran silently sitting in an ambulance have been shared thousands of times on social media.

“Many children are killed in this war when residential neighborhoods are attacked and they are at home or playing in the street,” said Mohammad, a surgeon in Aleppo who treated Omran and asked ABC News not to publish his last name due to safety concerns.

The ongoing conflict in Syria has caused the largest humanitarian crisis since World War II, with more than 8 million children in danger, according to the United Nations Children's Fund.

What began as a local uprising against Syrian President Bashar al-Assad slowly burgeoned into an international war involving the United States, Russia, Iran and almost all of Syria's neighbors. Military planes belonging to the Syrian regime and Russia, which began its military operations there against ISIS and other militant groups last September, regularly target rebel-held areas.

ABC News' Lena Masri contributed to this report.