U.S.-backed militias in Syria have captured an ancient citadel from Islamic State (IS) in a strategically significant advance against the jihadist group in its stronghold of Raqqa province, a spokesman said on Friday.

The Jabar citadel on the banks of Lake Assad was taken by the Syrian Democratic Forces (SDF) alliance on Thursday, militia spokesman Talal Silo said. It is located near a dam on the Euphrates River that the U.S.-backed alliance also aims to capture in the current phase of its campaign.

Maamoun Abdulkarim, the Syrian government’s antiquities chief, told Reuters in Damascus the SDF’s capture of the castle represented a “victory for the Syrian people because liberating the citadel from Daesh (IS) is saving Syrian heritage”.

The SDF, which includes the powerful Kurdish YPG militia, is the main partner for the United States in the campaign it is leading against Islamic State in Syria. SDF forces are just a few kilometres (miles) from a major dam also held by IS.

Islamic State is also being fought in separate campaigns by the Russian-backed Syrian army in Deir al-Zor province and near the ancient town of Palmyra, which IS seized for a second time in December, and by Turkish-backed Syrian rebels in Aleppo province near the Syrian-Turkish border.

Silo said SDF advances had been slowed by thick fog that had allowed Islamic State insurgents to use infiltration tactics to attack SDF positions, he said. The weather had now improved, he added in an interview over the internet.

“The direction of our forces is towards the area of the dam at present,” Silo said.

The SDF launched a multi-stage operation in Raqqa province in November aimed ultimately at capturing the city from Islamic State. The first phase gained territory to the north of the city and the current phase is targeting areas to the west of it.