A US-backed Kurdish-led alliance announced Tuesday it is launching the final stage of its battle to expel the Islamic State jihadist group from its desert holdouts in eastern Syria.

US backed Syrian Democratic Forces (SDF) said on Sunday they had expelled Syrian troops that had earlier taken control of a string of villages in oil rich areas it controls east of the Euphrates river near the border with Iraq.

A top Syrian Kurdish politician suggested on Wednesday that the Kurdish-led Syrian Democratic Forces (SDF) and the Iraqi government could mount joint operations against Islamic State in the area on the border of the two countries.

Hundreds of miles away from family and friends facing an attack by Turkey, Kurdish and Arab forces deployed in the eastern Syrian desert against jihadists have become increasingly frustrated.

US-backed militias in Syria said they have formed a joint military centre with the Iraqi army to protect their common border region after ousting Islamic State militants.

US-backed forces took full control of Raqqa from the Islamic State group on Tuesday, defeating the last jihadist holdouts in the de facto Syrian capital of their now-shattered "caliphate".

Among the ruins of a street in Syria's Raqqa, an anti-jihadist fighter's call echoes from a loudspeaker: "Civilians, we are waiting for you to leave to safety."

The US-backed Syrian Democratic Forces (SDF) said on Wednesday its campaign to capture Raqqa from Islamic State (IS) was in its final stages and its fighters had seized 80 percent of the city.

Intensified coalition air strikes supporting an assault by US-backed forces on Islamic State's stronghold of Raqqa in Syria are causing a "staggering loss of civilian life", United Nations war crimes investigators said on Wednesday.

A US-backed alliance of Arab-Kurdish forces entered the key jihadist-held town of Tabqa on Monday as they pursued their campaign against the Islamic State group in northern Syria.

US-backed fighters have advanced to the edge of a key jihadist-held town in northern Syria, a monitor said Saturday.

A US-backed alliance of militias has made unexpectedly rapid gains against Islamic State near its Syrian headquarters in Raqqa, advancing to within a few kilometres of a major dam, its spokesman said.