The PKK, which has waged an insurgency in Turkey for decades, and the YPG have both denied any involvement in the attack.

"The hour of reckoning has come," the Turkish defence ministry tweeted earlier Sunday, along with a photo of a plane taking off for a night operation.

"The treacherous attacks of the scoundrels are being held to account," it said.

"Terrorist hotbeds razed by precision strikes," the ministry said in another post, which was accompanied by a video showing a target being selected from the air followed by an explosion.

Turkey carried out more than 20 strikes on sites in the northern Syrian province of Aleppo and Hassakeh in the northeast, said the Syrian Observatory for Human Rights, a Britain-based monitoring group that has an extensive network of contacts across Syria.

The raids killed at least six members of the Kurdish-led Syrian Democratic Forces (SDF) and six pro-regime soldiers, the monitor said.

'Bombing threatens whole region'

While Ankara did not give details of the operation, Kurdish forces said the city of Kobane in northeast Syria was among the targets hit by Turkish raids.

"#Kobane, the city that defeated ISIS, is subjected to bombardment by the aircraft of the Turkish occupation," tweeted Farhad Shami, a spokesman for the SDF.

The SDF provided crucial assistance to a US-led coalition against jihadists of the Islamic State (IS) group.

But Turkey considers the Kurdish People's Protection Units (YPG) -- the main component of the SDF -- an extension of the outlawed PKK.

Turkish Interior Minister Suleyman Soylu has said Ankara believes the order for the Istanbul attack was given from Kobane, controlled by Syrian Kurdish militia forces.

"Turkish bombing of our safe areas threatens the whole region," Mazloum Abdi, the chief commander of the US-allied SDF, tweeted.

"This bombing is not in favour of any party. We are making every effort to avoid a major catastrophe. If war erupts, all will be affected."

Kobane, a Kurdish-majority town near the Turkish border, was captured by IS in late 2014 before Kurdish fighters drove them out early the following year.

The US State Department had said on Friday it feared possible military action by Turkey, advising its nationals not to travel to northern Syria and Iraq.

Turkey has launched waves of attacks on Syria since 2016 targeting Kurdish militias as well as IS jihadists, and Ankara and forces backed by it have seized territory along the Syrian border.

Since May, Turkish President Recep Tayyip Erdogan has threatened to launch a new operation in northern Syria.