Turkey has been bombing sites in the area since Thursday, hitting civilian and military targets and infrastructure and causing casualties, according to Kurdish authorities.

The Syrian Observatory for Human Rights, a UK-based monitor, said that 20 people had been killed and around 50 wounded after a Turkish war plane targeted a training centre belonging to Kurdish internal security forces, known as the Asayish, on the outskirts of Al-Malikiyah.

The Kurdish force acknowledged the strike, saying that "a number of our forces were killed and others wounded".

AFP correspondents said that authorities in the area have called for blood donations, while witnesses said that hospitals were full of casualties.

Amid the chaos of Syria's long-running civil conflict, Syria's Kurds have carved out a semi-autonomous area in the country's northeast.

Turkey's defence ministry said on Friday that it had launched the new wave of air strikes in retaliation for an attack in Ankara earlier this month that wounded two security personnel.

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.

Turkey launched strikes on PKK positions in northern Iraq hours after the October 1 attack, with foreign minister Hakan Fidan saying days later that the assailants "came from Syria and were trained there".

- Kurdish denial -

The US-backed, Kurdish-led Syrian Democratic Forces 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.

The SDF, the Kurds' de facto army in the northeast, denied that those behind the Ankara attack had passed through the area.

Turkish bombings had mostly subsided over the weekend after strikes hit energy infrastructure, including power stations and oil facilities on Thursday and Friday, killing at least 15 security personnel and civilians, according to the Kurdish authorities.

Since 2016, Turkey has carried out successive ground operations to expel Kurdish forces from border areas of northern Syria, and Turkish President Recep Tayyip Erdogan has threatened a new incursion.

Turkey supported early rebel efforts to topple Syrian President Bashar al-Assad, and maintains a military presence in northern stretches of the war-torn country which angers Damascus.

In November last year, Turkey launched air strikes on Kurdish-held areas of Syria and Iraq in response to a bombing in Istanbul that killed six people.

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