US-backed fighters have seized the last road into Raqqa and are moving eastward along the river south of the city, almost completing the siege on the Islamic State's de-facto capital, US officials and a Syria war monitor said Thursday.

A spokesman for the US-led coalition told The Associated Press that the Syrian Democratic Forces are now in control of all high-speed routes into Raqqa from the south. The Kurdish-led fighters had been advancing from the city's east after they seized a major stronghold in May, and from the west and north.

"South of the Euphrates river the SDF now control all high-speed routes into Raqqa," Col. Joe Scrocca, spokesman for the US-led coalition, said in emails to the AP. Moving toward the Euphrates from the east "would completely encircle the city and has been the SDF plan from the start."

The Britain-based Syrian Observatory for Human Rights said a unit of the SDF has seized villages across the river, moving through the Kasret al-Faraj area. The Observatory described it as a "strategic" advance that completes the siege around Raqqa.