The statement by France's Human Rights Defender Jacques Toubon came as controversy intensifies over the reluctance of French authorities to take in the children of French citizens affiliated with Islamic State (IS) jihadists in Syria and Iraq.

"The French state needs to adopt effective measures allowing the halt to the inhumane and degrading treatment of children and their mothers and put an end to the violations of the rights of the child," Toubon said.

He alleged violations of the UN Convention on the Rights of the Child, to which France is a signatory.

The human rights defender leads an independent state institution set up in 2011 and enshrined in the constitution to defend people whose rights have been violated.

Toubon had been asked to give an opinion on the issue by the lawyers of several families with French citizens held in Syria and Iraq.

According to the French foreign ministry, some 450 French citizens linked to IS are either detained by Kurdish forces in northern Syria or being held in refugee camps.

But France is reluctant to bring back foreign fighters or their families after suffering a wave of deadly jihadist attacks that have killed more than 250 people since 2015.

It has said it would consider requests for their return, but only on a case-by-case basis. Since March it has repatriated five orphans and a three-year-old girl whose mother was sentenced to life in prison in Iraq.

In April, France's State Council, which rules on the constitutionality of policies and laws, rejected several requests to intervene, saying it was a French diplomatic matter outside the council's jurisdiction.

"Finally a national authority has condemned France and urged it to respect its international and European obligations," said lawyer Marie Dose, who represents several of the affected families, after Toubon's statement.

Lawyers William Bourdon and Vincent Brengarth, who represented families who sought the opinion, welcomed the recognition "of the extreme seriousness of the violation of fundamental rights".

"We expect that the French authorities will organise the repatriation (of the children) as soon as possible," they said in a statement.

The grandparents of two children stranded with their French jihadist mother at a camp in Kurdish-held Syria filed a lawsuit with the European Court of Human Rights earlier this month over France's refusal to allow them home.