Syrian President Bashar al-Assad and a Kurdish-dominated force exchanged insults Monday, raising fears of a new fault line in the country's devastating seven-year conflict.

Assad labelled US-backed Kurdish fighters "traitors", prompting the Syrian Democratic Force to accuse his regime of flooding the country with "foreign terrorists".

"When we talk about those referred to as 'the Kurds', they are in fact not just Kurds," Assad said in remarks released by the presidency on social media. "All those who work for a foreign country, mainly those under American command... are traitors".

"This is how we see these groups working for the Americans," he told Russian media and Syrian officials after receiving a large Russian delegation.

His comments prompted an angry response from the Kurdish-dominated SDF, which seized swathes of northern and eastern Syria from the Islamic State group in a months-long assault backed by US-led coalition air support.

"Bashar al-Assad and what's left of his regime are the last people with the right to talk of treachery," the SDF said in a statement.

"It was the regime that flung the country's doors wide open to hordes of foreign terrorists from across the world."

The SDF also accused the regime of freeing "terrorists" from its prisons so they could "shed the blood of Syrians of all stripes".

The Syrian opposition has long accused Damascus of releasing extremists from its prisons in the early months of the uprising in a bid to turn the largely peaceful movement into an armed conflict.

- Uneasy face-off -

Assad had criticised the semi-autonomous Kurds in the past, but his latest comments were harsher than any since the war broke out in 2011 with demonstrations brutally crushed by state security forces.

The Kurdish People's Protection Units (YPG), which form the backbone of the SDF, have largely avoided armed clashes with the regime throughout Syria's war, which has killed more than 340,000 people dead and displaced millions since it broke out in 2011.

But the Kurds, who make up some 15 percent of Syria's population, were long oppressed under Assad and his father Hafez, and have long demanded greater rights.

Early on in the war, Kurdish forces adopted a position of neutrality towards both the regime and rebel forces.

Regime forces unilaterally withdrew from Kurdish areas in mid-2012 in a move widely perceived as a tactic to discourage the Kurds from allying themselves with rebels.

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

But while some in the regime have reached out to Kurdish forces, suggesting some level of autonomy could be eventually be discussed, senior figures have slammed the SDF for launching operations independently of Damascus.

With their common enemy now defeated across much of the country, the SDF and regime forces are in an uneasy face-off.

But the SDF denied it was seeking to break up the country.

"If Syrians do not face the regime, it will lead the country towards partition, which our forces will not allow in any form," it said.

Source: AFP