Turkey's president has called on Nato to take a stance against the US, a fellow ally, over its plans to form a 30,000-strong Kurdish-led border security force in Syria.

Turkey has been threatening to launch a new military offensive in Syria against Syrian Kurdish militias, which Ankara considers to be terrorists because of their affiliation with a outlawed group fighting an insurgency in southern Turkey.

On Monday President Recep Tayyip Erdogan accused Washington of creating an "army of terror" in Syria, along the border with Turkey, and vowed to crush the US-backed border force.

Addressing his ruling party's deputies on Tuesday, Mr Erdogan questioned Nato's stance on the issue, saying: "Hey Nato! You are obliged to make a stance against those who harass and violate the borders of your members."

Ties between Turkey and the US have deteriorated over the latter's support of the Kurdish militia, known as the People's Defense Units, or YPG, which Turkey says is a major threat to its security.