Islamic State militants have kidnapped some 500 children in the past few weeks and are most likely brainwashing and training them now to become child soldiers or suicide bombers to augment their ranks, Iraqi authorities warned.

The abducted children came from Anbar and Diyala provinces in Iraq and were taken to ISIS bases so they can be used in "terror attacks," something that the Islamic extremist group is known for, The Daily Mail reported on Tuesday.

The ISIS militants use children as their "caliphate cubs," who function as soldiers, executioners, or suicide bombers.

"They are going to brainwash these kids into being suicide bombers," Lieutenant General Kasim Al-Saidi, of the eastern province of Diyali, told Turkey's state-run media Anadolu news agency.

Around 100 children were taken from Diyali, while majority of the 500 were taken from Anbar, west of Baghdad, in a series of raids in the villages of Ar Rutba, Al-Qaim, Anah and Rawa.

Provincial council member Farhan Mohammed said the children were taken to bases in Iraq and Syria.

Reports last month revealed that the ISIS has been forcing kidnapped children from the minority Yazidi group to fight as jihadi soldiers or become suicide bombers.

"ISIS has established military training camps for the Yazidi children held by the group in the Syrian city of Raqqa and Tal Afar in Mosul," said Sheikh Shamo, a Yazidi member of the Iraqi parliament.

"Over the past months, many Yazidi women, children and elders managed to escape in various ways and have arrived in the Kurdistan region, but we still believe more than 3,000 Yazidis remain in the hands of ISIS," said the Yazidi lawmaker.

Propaganda by the ISIS includes videos of training camps where children are shown being groomed to become part of the group's terror campaign.

"They use dolls to teach them how to behead people. Then they make them watch a beheading, and sometimes they force them to carry the heads in order to cast the fear away from their hearts," an Iraqi official told NBC.

Meanwhile, two German twin brothers, age 25, who joined the Islamic extremists earlier this year have reportedly died in battle.

One of the twins, known by his Islamic name Abū Mus'ab al-Almānī, died after he drove his explosives-packed vehicle inside the camp of the 4th Regiment of the Iraqi army in Baghdad, reportedly killing dozens of soldiers, Opposing Views reported on Monday.

The other twin brother died in another suicide bombing.

The twins were recent converts of Islam. Details of the time of their travel to Syria to join the extremist group or whether German authorities have been watching them are still unclear.

Around 650 Germans have travelled to ISIS-controlled territories to fight side by side with the group.