Turkey has intensified its shelling of Kurdish militia in northern Syria, ahead of a threatened ground offensive aimed at driving them out of the area.

The Syrian Kurdish People’s Protection Units (YPG) said 70 shells were fired overnight.

Turkey has for months said it would clear YPG fighters from Afrin, under Kurdish control since 2012. Turkey regards the YPG as a terrorist group.

Syria has warned against an incursion, threatening to shoot down Turkish jets.

Turkish Defence Minister Nurettin Canikli said the shelling was the “de-facto start” of a planned invasion of Afrin.

Turkey’s military and intelligence chiefs are in Moscow to try to get Russia’s agreement to allow Turkish planes to use the Russian-controlled airspace above Afrin.

Russia’s acquiescence will be essential for any Turkish operation. It is a key ally of Syrian President Bashar al-Assad and has a contingent of soldiers at the airport in the centre of Afrin.

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Turkey has intermittently shelled and carried out air strikes against the YPG in the Afrin area, from where fighters have fired rockets into Turkey.

Ankara sees the militia as an extension of the banned Kurdistan Workers’ Party (PKK), which has waged a decades-long insurgency in south-eastern Turkey.

The YPG denies any direct links – an assertion backed by a US-led coalition whose air strikes and special forces have helped the militia and allied Arab fighters drive Islamic State militants from tens of thousands of square kilometres of Syria.

Turkish President Recep Tayyip Erdogan has repeatedly warned he will not allow a “terror corridor” linking Kurdish-controlled areas along the Syria-Turkish border.

Afrin is currently isolated from two other self-declared Kurdish autonomous cantons – Kobani and Jazira. Turkish-backed rebel forces took over a 100km- (60-mile) wide area separating the territories after driving out the Islamic State (IS) group in 2016.

Turkey has said the operation against Afrin will be a continuation of that seven-month offensive, known as Euphrates Shield, which ended in March 2017.

Turkey has been alarmed at the expansion of the Kurdish-controlled areas in northern Syria as the YPG-led Syrian Democratic Forces (SDF) emerged as one of the most effective forces fighting against IS.

US support for the SDF has infuriated Turkey, a fellow Nato member.

Ankara recently denounced a US plan to set up an SDF-led 30,000-strong border security force in Kurdish-controlled areas, purportedly to guard against a resurgence of IS.