Libya was in recent years split between a Government of National Accord in Tripoli, and an eastern-based administration, backed by strongman Khalifa Haftar.
"These girls and boys will be Syria's young adults in five or 10 years and will, in turn, want a future, economic prospects and political freedoms that the system cannot give them if it doesn't reform," he told AFP.
UNSMIL said it "calls on authorities to ensure an independent, impartial and prompt investigation into this heinous attack on civilians and calls for perpetrators to be brought to justice."
It marked the start of a new chapter for the country after the failure of a 2015 UN-brokered deal that established a Government of National Accord headed by Fayez al-Sarraj.
The oil-rich North African nation has been riven by civil war ever since the fall of Kadhafi, but rival sides finally agreed a fragile ceasefire last October.
While many victims and families are still waiting for recognition and compensation, few of those responsible have been held to account. Some now hold senior posts in the police or army.