DAMASCUS: Syrian President Bashar al-Assad called US-backed Kurdish fighters "traitors" Monday, ramping up the rhetoric against the forces controlling more than a quarter of the country.

"When we talk about those referred to as 'the Kurds', they are in fact not just Kurds. All those who work for a foreign country, mainly those under American command... are traitors," he said.

"This is how we see these groups working for the Americans," he said.

Assad had criticised the semi-autonomous Kurds in the past, but his latest remarks, released by the presidency on social media, were more virulent than usual.

The Kurdish minority accounts for an estimated 15 percent of Syria's population and the Kurdish People's Protection Units (YPG) control a large chunk of territory in the country.

Both Damascus, backed by Russia, and the Kurds, backed by a US-led coalition, have fought Daesh in recent months.

But their common enemy has been defeated across much of the country now, leaving the Kurdish-dominated Syrian Democratic Forces and regime forces in an uneasy face-off.

Some senior regime officials had in the past made overtures to the Kurds, suggesting some level of autonomy could be eventually be discussed, but Assad's latest comments augur poorly for any future talks.