The fathers of the children, young boys and girls between the ages of three and 10, are believed to have been killed in combat during Iraq's three-year war against the jihadists, the source said.

Chechen strongman Ramzan Kadyrov said earlier this week that he expected the children to arrive in Moscow on Sunday.

He said 24 of them were from Dagestan, and another three were from Chechnya.

Several thousand Russians travelled to join the jihadists in their once sprawling "caliphate" straddling Syria and Iraq, according to estimates from the Russian security services.

Some took their families with them.

Since last year, around 100 women and children -- mostly from Russia's Muslim-majority Caucasus -- have returned under a programme championed by Kadyrov.

But in mid-November, Chechen activist Kheda Saratova accused Russia's FSB security service of blocking attempts to bring back the remaining widows and children of Russian IS fighters.

"According to our organisation, there are over 2,000 of them left in Syria and Iraq," Saratova, who is on Kadyrov's human rights council, said at the time.

Meanwhile on Sunday Iraqi Prime Minister Adel Abdel Mahdi held talks in Baghdad with Anna Kuznetsova, the Russian president's envoy for the rights of children.

During the meeting, Abdel Mahdi said a "distinction should be made between humanitarian issues and terrorist crimes", according to a statement from his office.

"These children are also victims," he added.

More than 300 people, including around 100 foreigners, have been sentenced to death and many others to life imprisonment in Iraq for joining IS, the Sunni extremist group which at its peak controlled nearly a third of the country.

Baghdad declared victory against IS in December last year, but the jihadists maintain sleeper cells and have carried out periodic hit-and-run attacks.