Turkey and its rebel allies have opened up a new line of attack in northern Syria after Turkish tanks crossed the frontier from Kilis province, making a western thrust in an operation to sweep militants from its border.

Key points: The operation puts pressure on IS-held territory along Turkey's border

The operation puts pressure on IS-held territory along Turkey's border The US has struck IS targets in the area with a new rocket system

The US has struck IS targets in the area with a new rocket system Turkey aims to also clear the border of US-backed Kurdish fighters

The incursion from Kilis — an area repeatedly targeted by Islamic State (IS) rocket fire over the past year — coincided with a push elsewhere in the region by the Turkish-backed Syrian rebels, who seized several villages further to the east.

By supporting the rebels, mainly Arabs and Turkmen fighting under the loose banner of the Free Syrian Army, Turkey is hoping to push out IS militants and check the advance of US-backed Syrian Kurdish fighters.

The rebels last week took the frontier town of Jarablus with Turkish support as part of operation Euphrates Shield, Ankara's first full-scale Syrian incursion since the start of the five-year-old war.

On Saturday the tanks crossed the border and entered the Syrian town of al-Rai to support the new offensive, a rebel spokesman and monitors said.

Now under rebel control, al-Rai had previously been in the hands of IS.

Al-Rai is about 55 kilometres west of Jarablus, and part of a 90 kilometre corridor near the Turkish border that Ankara says it is clearing of jihadists and protecting from Kurdish militia expansion.

A rebel commander said they would aim to push east from al-Rai, in the direction of Jarablus, which would put pressure on Islamic State from both east and west of a stretch of territory it controls along the border between the towns.

"The operations are to work from al-Rai towards the villages that were liberated to the west of Jarablus," Colonel Ahmed Osman of the Sultan Murad rebel group told Reuters, adding that the offensive was backed by Turkey.

Additional villages captured, US deploys rocket system

The Hamza Brigade, also part of the Free Syrian Army, said it had taken control of Arab Ezza, a village about 30 kilometres west of Jarablus and near where Turkish warplanes carried out air strikes on Friday.

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Free Syrian Army factions had also captured the villages of Fursan, Lilawa, Kino and Najma just south of Arab Ezza, according to a source in another rebel group, the Failaq al-Sham.

The United States said it hit Islamic State targets in the region overnight, although it did not say where.

"US forces struck ISIL targets near Turkey's border in Syria last night via newly deployed HIMARS system," Brett McGurk, the special presidential envoy for the coalition fighting Islamic State, said on his Twitter account.

HIMARS refers to a High Mobility Artillery Rocket System.

Turkey has struggled to protect the area around Kilis from Islamic State rocket fire. Three rockets fired from northern Syria hit the region on Saturday, Dogan news agency reported, adding there were no casualties.

Erdogan still critical of US support for Kurdish fighters

While Euphrates Shield initially targeted Islamic State in Jarablus, most of the focus since has been on checking the advance of US-backed Syrian Kurdish fighters, to the alarm of NATO ally Washington.

Turkey disagrees with its ally's support for the Syrian Kurdish YPG militia, which it considers a terrorist group. The YPG has been among the most effective partners on the ground in the US-led fight against IS.

Turkey is worried that advances by Syrian Kurdish fighters will embolden Kurdish militants in its southeast, where it has been fighting an insurgency for three decades led by the outlawed Kurdistan Workers Party (PKK).

In China to attend the G20 meeting of world leaders, President Tayyip Erdogan said there should be no support for any terrorist organisation — a reference to the United States' backing of the Syrian Kurdish fighters.

"There is no good terrorist. All terrorists are bad. All organisations involved in terrorism are cursed," Mr Erdogan said.

"This is how we see things and how we put up our struggle."

The United States has voiced concerns about Turkish strikes on Kurdish-aligned groups that Washington supports. Germany said it did not want to see a lasting Turkish presence in an already tangled conflict.

Reuters