Turkeys Tayyip Erdogan threatened on Monday to strangle a planned 30,000-strong US-backed force in Syria before it is even born,as Washington's backing for Kurdish fighters drove a wedge into relations with one of its main Middle East allies. The United States announced its support on Sunday for plans for a border force to defend territory held by US-backed, Kurdish-led fighters in northern Syria. The Syrian government of President Bashar al-Assad responded on Monday by vowing to crush the new force and drive US troops from the country. Assads ally Russia called the plans a plot to dismember Syria and place part of it under US control. But the strongest denunciation came from Erdogan, who has presided as relations between the United States and its biggest Muslim ally within NATO have stretched to the breaking point.