Kurdish-aligned forces in northern Syria said Tuesday that a ceasefire had been reached with Turkey, after clashes that saw the main US ground ally against the Islamic State group pushed back by Syrian rebels backed by Turkish armour and airstrikes.

A spokesman for the US-led coalition against Islamic State forces could not confirm a ceasefire but said that the coalition welcomed "the calm that has been achieved" after the day passed without any evident hostilities between the two sides.

There was no official comment from Ankara, but rebel brigades involved in the Turkish-backed offensive inside Syria posted Twitter messages indicating they were planning to now move west along the border against Islamic State forces, rather than south against the Kurds.

The clashes had earlier drawn a rebuke from US Defence Secretary Ashton Carter, who called on Turkey to "stay focussed on the fight against [the Islamic State movement] and not to engage Syrian defence forces."

Turkey launched its forces last week into northern Syria. The move involved Ankara transferring hundreds of Syrian rebels from the west of the country to the northern border region to act as its main ground troops.

Backed by Turkish armour and airstrikes, the rebels quickly seized the key border town of Jarabulus from Islamic State fighters, but much of the ground they have taken since then was held by forces linked to the Kurdish-led, US-backed Syrian Democratic Forces (SDF).

The SDF recently captured the city of Minbij, south of Jarabulus, from Islamic State forces, a victory seen as key to breaking the extremist group's links to the Turkish border and the outside world and blocking movements of supplies and fighters deeper inside Syria and Iraq.

But the SDF advance also alarmed Turkey, which distrusts the Kurdish forces due to their links with Kurdish insurgents operating inside Turkey.

The small Sajur river north of Minbij now seems to have become an effective boundary between the SDF-linked forces and the Turkish-backed rebels. The SDF-linked Jarabulus Military Council announced late Monday that it had withdrawn south of the river.

The Jarabulus Military Council said that it had reached a temporary ceasefire agreement with Turkey, mediated by the US-led coalition, effective from midnight Monday.

Coalition spokesman Colonel John Dorrian told dpa he could not confirm a ceasefire. "There is clearly a calm between these forces, and there haven't been any [further] hostilities," he said.

In Washington, US State Department spokesman John Kirby said that for "12 to 18 hours or so" there had "been no clashes between those two sides, and that's a welcome development."

Rebel fighters allied with Ankara told dpa Tuesday they were refraining from attacks, for the time being, on Kurdish forces.

US Central Command chief General Joseph Votel said that "for the most part ... the portion of the Kurds that are part of the SDF" had "lived up to their commitment to us" and withdrawn from Minbij to the east of the Euphrates river, a key Turkish demand.

If the Kurds cannot stay west of the Euphrates, they will not be able to link up with an enclave they control in the far north-western corner and create a single autonomous region stretching across northern Syria. Rebels and Turkey oppose such a Kurdish entity.

The US is increasingly concerned that the clashes between its two allies could weaken the war on Islamic State forces.

US President Barack Obama and Turkish President Recep Tayyip Erdogan are set to meet at the weekend, on the sidelines of the G20 summit in China.

Erdogan had said the operation was "in cooperation with the international coalition" against Islamic State, which is led by the US. The coalition, however, does not include Turkish strikes in its daily toll, and the latest US remarks raise more questions.

The Wall Street Journal reported that the Turkish offensive in Syria was not fully coordinated with Washington and high-level talks broke down.

Erdogan vowed Monday that the offensive in northern Syria would continue until threats from both Islamic State forces and the Kurds were stopped.

US officials have, however, applauded the Turkish capture of Jarabulus, the last main town the extremists held on the border after the others were taken by the Kurds.

A clampdown by the Turks on the so-called "jihadi highway" into Syria, combined with the Syrian Kurds' gains in the north, has helped to drastically reduce the flow of foreign fighters to the extremist group.

Erdogan will also meet Russian President Vladimir Putin at the G20. Russia backs the Syrian government and is also carrying out strikes against groups in the country.

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