An Syrian Democratic Forces (SDF) fighter walks with his weapon in northern Raqqa province, Syria February 6, 2017. REUTERS/Rodi Said

BEIRUT (Reuters) - U.S.-backed Syrian militias will tighten the chokehold on Islamic State’s base in Raqqa, after cutting the last main road out of the city, a spokesman said on Tuesday.

The Syrian Democratic Forces (SDF) severed the highway between Raqqa and the jihadists’ stronghold of Deir al-Zor province on Monday, dealing a major blow to Islamic State which is under intense military pressure in both Syria and Iraq.

Islamic State is losing ground to three separate campaigns in northern Syria - by the SDF militias, by the Russian-backed Syrian army, and by Turkey and allied Syrian rebels.

The SDF advance means all main roads out of Raqqa are now cut. The U.S.-backed militias now plan to capture surrounding rural areas and advance towards the city to isolate it completely, SDF spokesman Talal Silo told Reuters.

In some areas, the militias were still up to 20 kilometers from the city limits, he added.

The SDF alliance, including the Kurdish YPG militia and Arab groups, launched a campaign in November to encircle and ultimately capture Raqqa from Islamic State, with air strikes and special forces support from a U.S.-led coalition.

“The frontlines have not reached the city yet, they are still in the countryside,” Silo said. “The campaign will go on until the city is completely cut off.”

The first two phases of the SDF campaign targeted areas to the north and west of Raqqa, with the third phase targeting areas to the east of the city.

Raqqa city is bordered to the south by the Euphrates River. The U.S.-led coalition against Islamic State has mounted numerous air strikes targeting the bridges over the river, the Syrian Observatory for Human Rights has reported.