The main Kurdish militia in Syria made a public promise in June to stop using child soldiers and halt their recruitment.
A total of 149 militia members under the age of 18 have since been demobilised, according to Elisabeth Decrey Warner, head of Geneva Call.
But there are still believed to be up to 100 under-age fighters in the ranks of the YPG (People's Protection Units) and its women's arm, the YPJ (Women's Protection Unit).
Decrey Warner said that YPG and YPJ signed a formal pledge on Saturday, along with the self-declared government of Syrian Kurdistan, not to accept under-age recruits in the future.
More From This Section
"We undertake to no longer use any under-18 children in hostilities, including for combat, spying, guarding tasks or supplying combattants, and we won't admit any under-18s as combatants in our ranks," Kurdish defence official Abdulkerim Sarukhan said in a Geneva Call statement yesterday.
Their forces' combined strength is estimated at up to 30,000, protecting territories with 2.5 million residents and 500,000 people who have fled other parts of Syria.
"We've been negotiating for months over the presence of children in their ranks," Decrey Warner told reporters.
She and other Geneva Call representatives travelled from Iraqi Kurdistan to sign the accord, and staff members remain there to monitor its implementation.
Turkey's PKK Kurdish rebels made a similar pledge last October, and Geneva Call has also inked deals with Kurdish groups in Iran.
Geneva Call's programme director, Mehmet Balci, said it had put out feelers to a range of rebel groups in Syria from 2012 onwards, including the mainstream Free Syrian Army.
The number of child soldiers in Syria is unknown, but the Violations Documentation Centre, a organisation close to the opposition-in-exile, reported that 194 "non-civilian" children have been killed since September 2011.