ISTANBUL: Turkey warned Monday it would carry out more strikes on a Syrian Kurdish militia if it failed to retreat beyond the Euphrates River, as Washington voiced alarm over weekend clashes.

Turkish forces pressed on with a two-pronged operation inside Syria against the militant Islamic State (IS) and the Syrian Kurdish People's Protection Units (YPG), shelling over a dozen targets.

The strikes against the YPG are hugely sensitive as the outfit -- seen as a terror group by Ankara -- is allied with Turkey's NATO partner, the United States, in the fight against IS in Syria.

Ankara said it killed 25 Kurdish “terrorists” in strikes on YPG positions on Sunday, a day after a Turkish soldier died in a rocket attack.

The Pentagon called the clashes “unacceptable” and urged an immediate de-escalation.

Turkey's operation aims to push the YPG back across the Euphrates to prevent it joining up a region east of the river already under its control with a Kurdish-held area to the west.

'Ethnic cleansing'

Ankara fears the emergence of an autonomous Kurdish region in Syria would bolster Kurdish rebels across the border in southeast Turkey.

US Vice President Joe Biden said last week Washington had ordered the YPG to retreat or risk losing American support. But Ankara says it had seen no evidence of this.

“The YPG... needs to cross east of the Euphrates as soon as possible. So long as they don't, they will be a target,” said Foreign Minister Mevlut Cavusoglu.

Cavusoglu accused it of “ethnic cleansing” in the mainly Arab area around the city of Manbij, west of the Euphrates, which the YPG wrested from IS earlier this month.

Turkey accuses the YPG of being an offshoot of the Kurdistan Workers Party (PKK) which has waged a deadly insurgency on Turkish territory for over three decades.

On Monday, the Turkish air force launched air strikes on PKK bases in northern Iraq, state media said.

'Deep concern'

The Britain-based Syrian Observatory for Human Rights said at least 40 civilians were killed in Turkish shelling and air strikes on pro-Kurdish positions in northern Syria on Sunday.

Ankara strongly denies killing any civilians.

The army said Monday 10 more villages had “been cleared of terrorist elements” by Turkish-backed anti-regime Syrian fighters.

A 400-square-kilometre area has been cleared since the operation began on August 24, it added.

President Recep Tayyip Erdogan said in a statement the offensive would continue until “the threat of Daesh and YPG/PKK is over.” Daesh is an acronym for IS.

On Monday, at least five people were injured in the Turkish town of Kilis by rockets fired across the border from an IS-held area, NTV television reported. The army returned fire, the report said.

Ankara-backed forces faced little resistance when they captured the IS border stronghold of Jarabulus last week, days after a suspected IS suicide bombing killed 55 people in southeast Turkey.

But the standoff with the Kurdish militia has been intense, with a Turkish soldier killed on Saturday in a YPG rocket attack on his tank.

Pentagon spokesman Peter Cook said the clashes were “unacceptable and a source of deep concern”.

“The United States was not involved in these activities, they were not coordinated with US forces, and we do not support them,” he said.

He called for steps to de-escalate the situation and said Washington had once again told the YPG to retreat east of the Euphrates. This has “largely occurred,” he added.

'Prevent Kurdish corridor'

The Jarabulus military council -- affiliated to the YPG-dominated Syrian Democratic Forces (SDF) -- said its forces had withdrawn south of the Sajur River, 20 kilometres south of Jarabulus, “to protect the lives of civilians.”

This goes some way to meeting Turkish demands but it still leaves SDF-affiliated forces in and around Manbij.

The Turkish army said it had fired 61 times on targets in northern Syria in the last 24 hours. It did not say which group was targeted.

Deputy Prime Minister Numan Kurtulmus confirmed one of the key aims of its operation was to prevent the creation of a Kurdish-controlled corridor stretching from Iraq to the verge of the Mediterranean.

“If that happens, it means Syria has been divided,” he added, quoted by NTV television.

He said all relevant parties had been forewarned of Turkey's operation in Syria, including the regime of President Bashar al-Assad, a bitter enemy of Ankara who had been informed by its ally Russia.

Syria's official Sana agency said Damascushad written to the UN Security Council to protest Turkey's “crimes against humanity” in the country.