Western states have provided counter-terrorism training in Syria to Kurdish security forces that are battling Islamic State, the head of the force said, in a significant expansion of support to the Kurdish administration in northern Syria.

The internal security force, known as the Asayish, is part of the Kurdish administration that has emerged in areas of northern Syria where the Damascus-based state has lost control since the eruption of the Syrian war in 2011.

The Kurds have proven the only effective partner on the ground in Syria for a U.S.-led air campaign against Islamic State, having seized wide areas of territory from the jihadists this year near the border with Turkey. […]