Turkey's army and its allies have thrust deeper into Syria, seizing territory controlled by Kurdish-aligned forces on the fifth day of a cross-border campaign that a monitoring group said had killed at least 35 villagers.

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 airstrikes 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There was no immediate comment from the YPG, but forces aligned with the group have said it had withdrawn from the area prior to the assault.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 Wednesday to support its Syrian rebel allies.

The Turkish-backed forces first seized the Syrian border town of Jarablus from so-called Islamic State militants before pushing south into areas held by Kurdish-aligned militias. They have also moved west towards IS 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 IS 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 IS, 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.

The Observatory, a Britain-based monitoring group with a network of sources in Syria, said Turkish-allied forces had seized at least two villages south of Jarablus, Jub al-Kousa and al-Amarna, that were held by militias loyal to the SDF.

The fighting killed 20 civilians in Jub al-Kousa and 15 in al-Amarna, while scores more were wounded, the group said.