"Six Belgian orphan children were handed over to a delegation from the Belgian foreign ministry," spokesman Kamal Akef told AFP.

"The children and Belgian delegation have arrived in their country."

Top foreign affairs official Abdulkarim Omar, using an alternative acronym for IS, earlier said the "children from ISIS families" were repatriated "at the request of the Belgium government".

Belgium had announced the transfer of the children from Kurdish-controlled camps in Syria after the deaths of their jihadist parents.

"These are children who were born in our country and who today no longer have parents," Belgian Finance Minister Alexander De Croo told VRT public radio.

Belgium is one of several European countries wrestling with the dilemma of what to do about citizens trapped in Syria following the defeat of the IS.

Some are reticent to accept captured extremist fighters, but the cases of children and non-combatant wives have proved more complicated for Western authorities.

On Wednesday, Foreign Minister Didier Reynders said Belgium has signed a deal to allow returnees to transit through Iraq's autonomous Kurdish region.

The bulk of IS fighters and family members who were captured when its so-called caliphate collapsed are being held in Syria in Kurdish-run camps.

De Croo said that four of the six orphan expected to return were older than 10, but that none were suspects.

"These are children who were alone in the camps, who have no support. There's no question of taking back parents who chose to join terrorist groups," he said.

"These children had no choice," he added.

According to Belgian media reports, 50 to 60 Belgian children under 18 are in the camps of Al-Hol, Roj, and Ain Issa in Syria.

Belgium was one of the European countries which, relative to its size, saw one of the larger contingents of extremists set off for the Syrian battlefield.

Authorities estimate that 400 adults have headed to jihadist-controlled areas since 2012 and 150 were still considered "active and in place" at the end of last year.

Syria's Kurds have detained hundreds of foreigners suspected of fighting for IS, as well as thousands of related women and children, during the US-backed battle against IS in Syria.

The Kurdish-led Syrian Democratic Forces expelled the extremist group from its last patch of territory in the eastern Syrian village of Baghouz in March, after larger than expected numbers of families emerged from the ruins.