Turkish forces crossed the border into Syria's Afrin district on Sunday, Prime Minister Binali Yildirim has said.

At a news conference in Istanbul, he said Turkey's military aimed to create a security zone some 30 kilometers (18 miles) inside the war-ravaged country.

The state-run Anadolu news agency also reported the arrival of Turkish forces in the enclave as part of an operation codenamed Olive Branch, adding that airstrikes and artillery shelling that targeted the area, which began on Saturday, were continuing.

Turkish President Recep Tayyip Erdogan said he hoped the offensive would be completed in "a very short time."

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YPG denies reports

Turkey's claims were immediately denied by a Kurdish People's Protection Units, or YPG, spokesman, who said Turkish forces and their rebels tried to cross into the province but failed, after fierce clashes erupted.

The Britain-based Syrian Observatory for Human Rights (SOHR) reported fighting between the two sides on the northern and western edges of Afrin, but said that Turkish troops had failed to advance.

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Although Ankara has not released figures on the number of Turkish troops involved in the offensive, SOHR's director Rami Abdurrahman said some 10,000 Syrian fighters have been readied. A rebel commander quoted by Reuters news agency on Sunday said around 25,000 Free Syrian Army rebels would also back Turkey in its campaign to recapture YPG-held towns and villages.

The apparent ground assault comes a day after Turkey launched an air assault against the Syrian-Kurdish YPG militia, which controls the northwestern corner of Syria, and which Ankara regards as a terror group linked to a more than three-decade insurgency in its southeast.

Turkey's pro-Kurdish Peoples' Democratic Party (HDP) has called on Kurds in Turkey to protest against the military incursion. But President Erdogan warned Sunday at a congress of his ruling AK Party that anyone who took to the streets would "pay a high price." 

"This is a national struggle. We will crush anyone who opposes our national struggle," he said.

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Saturday's airstrikes and artillery fire were backed by pro-Turkey Syrian rebels who were engaged in a "comprehensive" ground operation against the YPG, Andadolu said.

SOHR said eight civilians, including a child, were killed in the strikes on Afrin.

Rockets hit Kilis

Early on Sunday, four suspected YPG rockets struck the central-southern Turkish town of Kilis from across the Syrian border, the town's governor said.

Governor Mehmet Tekinarslan said the rockets hit two houses and an office, slightly injuring a woman, before Turkish artillery returned fire.

"No one lost their life," Tekinarslan was cited by the Dogan news agency as saying. "They can fire one rocket at us and we will fire 100 back. There is no need to worry."

A missile also hit the Turkish border town of Reyhanli, killing one person and wounding more than 30, the town's mayor told broadcaster NTV.

While Erdogan has vowed to crush the Kurdish militia, the YPG is the major force within the US-backed Syrian Democratic Forces alliance, which is leading the fight against the "Islamic State" (IS) militant group in Syria.

Syrian President Bashar Assad on Sunday condemned the Turkish offensive, saying it was supporting "terrorists" — a term the Syrian regime often uses to describe opposition forces in the country. The Syrian government last week warned that it could shoot down any Turkish warplanes used in an assault. 

nm, mm/jlw (AFP, AP, dpa, Reuters)