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The grandparents of two children stranded with their French militant mother at a camp in Kurdish-held Syria filed a lawsuit at Europe's top rights court Monday over France's refusal to allow them home, lawyers said.

It was the latest challenge to the French government's opposition to returning the children of suspected militants in Syria or Iraq.

The four-year-old boy and three-year-old girl, who were born in Syria, are among an estimated 500 children of French citizens who joined the Islamic State's so-called "caliphate" before the militants' last Syrian redoubt was overrun in March.

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

Critics say the policy exposes innocent victims of the war, many of whom have suffered serious trauma during the fighting and coalition bombardments, to inhumane living conditions and long-term psychological risks.

The grandparents of the two children, whose mother had joined militants in Syria, filed a complaint with the European Court of Human Rights (ECHR) in Strasbourg, eastern France, their lawyers told AFP.

"By refusing to repatriate this mother and these two sick children who are wounded and in a state of extreme weakness, France is consciously and deliberately exposing them to inhumane and degrading treatment," they said in a statement.

The suit also accuses France of violating the family's right to enter the state where they are citizens - the first time such an argument has been invoked in the case of militants' children, the lawyers said.

They declined to name the mother.

Some Western women have given birth to children in territory previously controlled by IS, but their children have never been registered with authorities in their home countries.

'Justice on French soil' 

The two children and their mother, who is the subject of an arrest warrant by a French anti-terror judge, were wounded in the battle for Baghouz, the last IS stronghold taken by Kurdish-led forces.

They are being held at the camp of Al-Hol in northeast Syria, where 73,00 people live according to the United Nations, including 12,000 foreigners.

The lawyers for the family said cholera, tuberculosis and dysentery are spreading in the camp, with the children undernourished and the mother suffering from typhoid fever.

The mother "has confirmed her desire to have her children repatriated... and wants to face justice on French soil," her lawyers said.

But France is loathe 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.

After considering repatriation at the urging of Kurdish forces in Syria as well as the United States, the government decided against bringing home any men or women who joined the IS fight.

Legal attempts beginning last year to force France to allow the return of citizens held in Syria have failed.

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

Former president Francois Hollande has agreed to meet with relatives of children in camps on Tuesday, lawyers said.

They hope he will "hear the distress" of those "who should not have to pay for their parent's choices," a statement issued by Marie Dose and Henri Leclerc added.

'Hunger and cold'

Repatriation of citizens who went to fight in Syria and Iraq is a hugely sensitive issue in Western nations.

Britain and the United States have even stripped some Islamic State proponents of citizenship.

This hardline stance drew criticism in March when 19-year-old Shamima Begum lost her British citizenship after giving interviews in which she begged to return home but showed little remorse for having been an IS propagandist.

Her baby Jarrah died of pneumonia less than three weeks after he was born.

The burden of treating children is also daunting for Western governments, which have had to set up special services for dealing with deep psychological scars from their years in the battle zones.

Many show the symptoms of post-traumatic stress disorder, including vivid nightmares, mutism and a visceral rejection of any attachment to others.

"These children have known hunger and cold. Some had already been separated from their parents, sometimes to protect them from the war, and they are scared of being abandoned again," said Marie-Rose Moro, a doctor who has treated several returned families.