On Saturday, Turkey announced a new wave of air strikes in retaliation for two separate attacks on its bases in northern Iraq that killed 12 soldiers, which Ankara blamed on Kurdish militants.

"Six civilians have been killed in separate Turkish air strikes," said Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights.

The head of the Britain-based monitor, which has a network of sources inside Syria, said four of the victims were employees of a printing press in the northern city of Qamishli, near the Turkish border.

"A strike on a mill near Qamishli killed a civilian while another was killed in a gas storage facility," he added.

The strikes hit about a dozen targets, all of them facilities run by the semi-autonomous Kurdish administration, the monitor and AFP correspondents in the region said.

Syrian Kurdish news agency ANHA also reported six deaths, four of them at the printing press, adding that six more people had been injured.

On Saturday evening, an AFP correspondent as well as the Observatory reported strikes against oil sites near the Turkish border, without reporting any victims.

In October, Turkey intensified air strikes on Syria's northeast after an attack in Ankara that wounded two security personnel earlier that month.

A branch of the Kurdistan Workers' Party (PKK) -- listed as a terror group by Ankara and its Western allies -- claimed responsibility for the first bombing to hit the Turkish capital since 2016.

The US-backed, Kurdish-led Syrian Democratic Forces (SDF) spearheaded the battle to dislodge Islamic State group fighters from their last scraps of territory in Syria in 2019.

Turkey views the Kurdish People's Protection Units (YPG) that dominate the SDF as an offshoot of the PKK.

Since 2016, Turkey has carried out successive ground operations to expel Kurdish forces from border areas of northern Syria.

The conflict in Syria has killed more than half a million people since it began in 2011 with the authorities' brutal crackdown on anti-government protests, spiralling into a devastating war involving foreign armies and jihadists.