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 says has killed at least 35 villagers.

Key points: Security sources say sites held by Kurdish YPG militia targeted

Security sources say sites held by Kurdish YPG militia targeted Turkey says 25 Kurdish militants killed

Turkey says 25 Kurdish militants killed Dozens of civilians said to be killed in towns of Jub al-Kousa and al-Amarna

Turkish warplanes roared into northern Syria at daybreak and its artillery pounded what security sources said were sites held by Kurdish YPG militia, after the Syrian Observatory for Human Rights reported fierce overnight fighting around two villages.

Turkey's military said 25 Kurdish militants were killed in its air strikes.

There was no immediate comment from the YPG, but forces aligned with the Kurdish militia have said it withdrew from the area targeted by Turkey before the offensive.

Turkish President Recep Tayyip Erdogan, speaking at a rally in the southern city of Gaziantep, near the Syrian border, vowed to devote equal energy to combating Islamic State jihadists and Syrian Kurdish fighters.

"We will make any kind of contribution to the work to clear Daesh (IS) from Syria," he told the rally.

"For the issue of the PYD (Democratic Union Party) terror group in Syria, we have just the same determination," he added, referring to the main pro-Kurdish party in northern Syria and its YPG militia.

Turkey, which is also battling Kurdish insurgents on its own soil, 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 Islamic State militants before pushing south into areas held by Kurdish-aligned militias. They have also moved west towards Islamic State areas.

Turkish officials have openly stated that 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, Turkey's offensive has so far focused mostly on targeting forces allied to the Kurdish-backed Syrian Democratic Forces (SDF), a coalition that includes 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.

So Turkey's action against SDF-allied forces puts it odds with a fellow NATO member, adding a further twist to Syria's complex war that began in 2011 with an uprising against President Bashar al-Assad and has 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 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 injured, the group said.

Turkish soldier killed

The Observatory said rebels backed by Turkish tanks fought until dawn against rival militias allied to the SDF around al-Amarna. SDF-allied militia damaged three Turkish tanks, it said.

Turkish security sources said warplanes and artillery had hit Kurdish YPG militia sites south of frontier town of Jarablus and towards Manbij, a city captured by Kurdish-aligned SDF this month in a US-backed operation.

President Erdogan told a rally that he would battle IS and the YPG with the "same determination". ( Reuters: Umit Bektas )

Colonel Ahmed Osman, head of the Turkish-backed Sultan Murad rebel group, said the force was "certainly heading in the direction of Manbij" and hoped to take it days. He also said Turkish-backed rebels were pushing west against Islamic State.

The Ankara Government wants to stop Kurdish forces gaining control of an unbroken swathe of Syrian territory on Turkey's frontier, which it fears could embolden the Kurdish militant group PKK that has waged a three-decade insurgency in Turkey.

A Reuters witness in Karkamis, a Turkish border town, heard jets and artillery bomb Syrian targets. A Turkish official told Reuters even heavier air strikes could follow soon.

Turkey said one of its soldiers was killed on Saturday when a rocket hit a tank that it said came from a YPG-controlled area. It was the first Turkish death reported in the campaign.

Turkey has suffered shock waves from the conflict raging in its southern neighbour, including frequent bomb attacks by Islamic State. The Government suspects the jihadist group was behind a blast at a wedding this month that killed 54 people.

Mr Erdogan visited Gaziantep where the attack happened to pay his respects to families of the victims.

Reuters