More than 65,000 have vanished in Syria since 2011, according to The Syrian Network for Human Rights.
Cluster bomb attacks on rebel-held areas of Aleppo have killed nine civilians and sent thousands of others fleeing.
The widely proscribed cluster munitions were used in the bombardment of rebel-held areas of Aleppo’s Masshad, Salaheddine and Sukkary after midday prayers on Friday.
Those killed were women and children on a minibus, trying to leave the areas hardest hit by the strikes. At least 20 more people have been wounded in the attacks.
Images obtained by Al Jazeera show unexploded cluster bomblets lying around the city as civilians walk nearby.
The use of the weapons, which scatter hundreds of bomblets over a vast area, has forced thousands of residents to take the risky journey out of the city through areas of intense fighting between government troops and rebel fighters.
Eliot Higgins, a journalist who specialises in the analysis of munitions used in the Syrian conflict, said the bomblet pictured in one image was the ShOAB-05 – a Russian-produced munition.
The bomblet contains a 70-gramme mixture of TNT and RDX explosives lined with more than 304 steel pellets, according to a blog post by munitions expert N R Jenzen-Jones.
Lacking the means to defuse the explosives, rebel fighters in Aleppo are trying to detonate the bomblets from a distance by taking pot shots at the munitions with assault rifles.
One child was seen riding a bicycle over an unexploded bomblet, stumbling and injuring himself without the device detonating.
British aid worker Tauqir Sharif posted a video of six young children being rushed to hospital with injuries sustained during a cluster bomb attack.
“So many people are dying that I’ve lost count. I have seen so many dead in the past five days that every time I close my eyes I see their faces,” Sharif, whose ambulance was damaged in a separate attack, said.
The use of cluster bombs is banned by more than 100 countries which cite the weapon’s inability to distinguish between combatants and civilians, as well as the large amount of unexploded bomblets that are left behind.
Human Rights Watch has condemned the Russian and Syrian governments for their use of the weapons and has documented at least 47 cluster bomb attacks in three provinces between May 27 and July 25 this year.
“Since Russia and Syria have renewed their joint air operations, we have seen a relentless use of cluster munitions,” HRW’s Ole Solvang said in a report published in July.
Neither Syria nor Russia have ratified the 2008 Convention on Cluster Munitions, which prohibits the use of the weapon.
Syrian government forces and their Russian allies have intensified aerial attacks on Aleppo after opposition fighters broke a siege of rebel areas of the city earlier in August.
Aleppo was Syria’s largest city and commercial centre before the start of the conflict in 2011.
Syrian rebels control a swath of territory that includes the city’s eastern and southern areas, as well as the surrounding countryside.
The Syrian government controls the city’s west as well as a northern corridor connecting it to government territories.
Syria’s war has claimed at least 400,000 lives and forced millions from their homes.