A leading international rights group on Wednesday accused Syria's main Kurdish militia of continued violations of the ban on child soldiers, despite some progress to stop using boys and girls under the age of 18 in fighting.

The militia has been the main force fighting the Islamic State group in Syria and captured wide areas of northern Syria along the border with Turkey from the extremists this year.

Human Rights Watch said the Kurdish People's Protection Units, or YPG, and its women's branch, YPJ, are not the only offenders among Syria's armed groups.

"The YPG promised to stop sending children to war and it should carry out its promise," said Fred Abrahams, HRW's special adviser, adding that fighting militants such as the Islamic State group is "no excuse to tolerate abuses by its own forces."

In 2014, YPG signed a "Deed of Commitment" with the non-governmental organization Geneva Call, pledging to demobilize all fighters under 18 within a month.

HRW said that it compiled a list of 59 children -- 10 of them under 15 -- who were recruited by or volunteered for YPG or YPJ since July 2014.

After asking the YPG in a letter for a response to the allegations, the group replied that it faces "significant challenges" in stopping its use of child soldiers in the ongoing armed conflict.

HRW noted that, on June 13, the YPG demobilized 27 boys and a week later the YPJ demobilized 16 girls. Seven YPG officers had been punished for accepting child soldiers and three were expelled from the force and four demoted, HRW said.