TRIPOLI (Reuters) - Forces aligned with Libya's new unity government advanced on the eastern and southern outskirts of the Islamic State stronghold of Sirte on Wednesday, taking control of at least one military camp, security sources said.

The brigades, who are based in the western city of Misrata, launched their counter-offensive against Islamic State last month, pushing the militants back along the coastal road to the west of Sirte and saying they intended to recapture the city.

The brigades are aligned with the U.N.-backed Government of National Accord (GNA), which has been trying to establish its authority over Libya's competing political and armed factions since arriving in Tripoli in March.

After a lull in fighting at the start of the week, clashes resumed in Sirte on Wednesday and the brigades edged forward, capturing the Taqrift military camp, security sources said.

The brigades' media office said in statements on social media that they had also captured the Al-Jalet military camp and the Au Hadi roundabout immediately south of Sirte, though this could not immediately be confirmed.

A hospital spokesman in Misrata said at least six brigade members had been killed and 30 wounded in the clashes.

Libya slid into chaos after veteran ruler Muammar Gaddafi was toppled in an uprising five years ago, allowing Islamic State to set up its most important base outside Syria and Iraq.

The GNA is designed to replace two competing administrations that were set up in Tripoli and the east in 2014 with the backing of rival alliances of armed groups.

Western powers see the GNA as the best hope for defeating Islamic State in Libya.

(Reporting by Ahmed Elumami and Ayman al-Warfalli; Writing by Aidan Lewis; Editing by Gareth Jones)