Turkish ground troops have crossed into northern Syria, with President Recep Tayyip Erdogan vowing to crush Kurdish fighters “very quickly”.

The Kurdish YPG militia says it has repelled Turkish troops in Afrin, and is shelling Turkish border areas.

Turkey considers the YPG a terrorist group, but it forms a crucial part of a US-backed alliance battling Islamic State (IS) jihadists in Syria.

The US has urged Turkish “restraint” in order to avoid civilian casualties.

It is the second day of Turkey’s military operation “Olive Branch”.

Turkish troops, accompanied by pro-Turkey rebels from the Free Syrian Army (FSA), have already advanced some 5km (3 miles) into Syrian territory, state-run Anadolu news agency reported.

Turkish Prime Minister Binali Yildirim said the aim was to establish a 30km (19-mile) “safe zone” deep inside Syria.

But a spokesperson for the YPG, Nouri Mahmoudi, said the group had managed to repel Turkish troops and they were “forced to retreat”.

Some 25,000 FSA fighters have joined the offensive on the Turkish side, rebel commander Maj Yasser Abdul Rahim told Reuters. It is not clear how many Turkish soldiers are on the ground.

Turkey’s military said it had hit 45 targets on Sunday, as part of its a