Turkish armoured personnel carriers drive towards the border in Karkamis on the Turkish-Syrian border in the southeastern Gaziantep province, Turkey, August 27, 2016. 
Turkish warplanes roared into northern Syria at daybreak and artillery pounded what security sources said were sites held by the Kurdish YPG militia, after the Syrian Observatory for Human Rights reported fierce overnight fighting around two villages.

Turkey said 25 Kurdish militants were killed in its air strikes and denied there were civilian casualties.

There was no immediate comment from the YPG, but forces aligned with the group have said it had withdrawn from the area prior to the assault.

Turkey, which is also battling Kurdish insurgents at home, sent tanks and troops into Syria on August 24 to support its Syrian rebel allies. The Turkish-backed forces first seized the Syrian border town of Jarablus from Islamic State militants before pushing south into areas held by Kurdish-aligned militias. They have also moved west towards Islamic State areas.

Turkish officials say their goal in Syria is as much about ensuring Kurdish forces do not expand the territory they already control along Turkey's border as it is about driving Islamic State from its strongholds.

However, the Turkish offensive has so far focused on forces allied to the Kurdish-backed Syrian Democratic Forces (SDF), a coalition that includes the YPG, an Observatory source said.

The SDF has support from the United States - which sees the group as an effective Syrian ally against Islamic State, putting Turkey at odds with a fellow NATO member and further complicating Syria's five-year-old civil war.

The conflict began as an uprising against Syrian President Bashar al-Assad and has since drawn in regional states and world powers.