BEIRUT (Reuters) - U.S.-backed Syrian rebels fighting Islamic State will announce the start date for the final assault on Raqqa city on Tuesday, aiming to oust the militants from their de facto capital in Syria, the Syrian Democratic Forces (SDF) said on Monday.

The SDF is an alliance of Kurdish and Arab militias backed by the U.S.-led coalition against Islamic State. On Saturday the Syrian Kurdish YPG militia, part of the SDF, told Reuters the fifth and final stage of the attack would start in the coming few days.

“The SDF will announce tomorrow the date of the start of the fifth stage of the campaign to liberate Raqqa which will include an assault on the city of Raqqa,” the SDF said in a statement.

In the past week the SDF has advanced to within a few kilometers of the city’s edge, after gradually encircling Raqqa since November in a multi-phased campaign to drive Islamic State from the city where it has planned attacks on the West.

On Monday it took some of the final few villages to the west of the city which is located on the River Euphrates some 90 km (56 miles) from the Turkish border, the Syrian Observatory for Human Rights war monitor said.

“The (SDF) will enter the city and begin attacking Raqqa directly,” Ibrahim Ibrahim, head of the Syrian Kurdish PYD party’s media centre in Europe, told Reuters. The PYD is the political arm of the YPG.

The assault on Raqqa will pile more pressure on Islamic State’s self-declared caliphate with the group facing defeat in the Iraqi city of Mosul and being forced into retreat across much of Syria, where Deir al-Zor is its last major foothold.