Turkish mortars killed one 12-year-old boy and ripped off a young girl’s leg as Turkey pummeled Kurdish-held towns on the second day of its assault on northern Syria on Thursday.

Muhammad Yusuf Hussein and his 7-year-old sister Sarah were hit in a strike on Qamishli, the de facto capital of the unrecognised Kurdish statelet of Rojava.

On the other side of the border, Turkish authorities said four children including a nine-month old baby had been killed in retaliatory fire.

They were the youngest of at least 15 civilians killed in artillery and airstrikes in Turkey's bid to create a "safe zone" in a large swathe of territory controlled by the Syrian Democratic Forces, a Kurdish-led militia.

Turkey considers the Kurdish YPG, the dominant force in the SDF, to be an extension of the Kurdistan Workers Party (PKK), which has fought a decades long insurgency against Ankara and is considered a terrorist organisation by Turkey and Nato allies including Britain.