BENGHAZI, Libya (Reuters) - Fighting between the self-styled Libyan National Army and rival armed groups in Benghazi killed six troops and wounded eight as the LNA advanced into the last pockets of resistance in eastern Libya’s largest city, a commander said on Saturday.

The LNA, led by Khalifa Haftar, also carried out air strikes on armed groups in the city of Derna, further east, who fired back with anti-aircraft cannon, witnesses told Reuters.

Haftar’s eastern forces are engaged in a multi-sided conflict between factions that fought together to oust veteran Libyan leader Muammar Gaddafi in 2011 but are now battling each other.

A U.N.-backed government in Tripoli is trying to bring stability to the North African OPEC member, but it is struggling to impose its authority. Haftar has so far rejected the Tripoli government as his forces advance in eastern Libya.

Two weeks ago, Haftar declared Benghazi’s “liberation” in a televised speech after three years of fighting. But street battles continue as his LNA troops face resistance from the Majlis Shura Thuwar Benghazi, an alliance of ex-rebels and Islamists.

“Special forces carried out an attack on Friday on the last resistance pockets of terrorism in Khraibeesh area and successfully the forces advanced and controlled several points,” an LNA commander, Mari El-Houti, said of the Benghazi operation.

El-Houti said the advance was impeded by a large number of mines in the area. He said 30 mines were dismantled on Friday.

Haftar, a former Gaddafi ally who turned against him, has gained ground with support from Egypt and the United Arab Emirates. Derna, some 380 km (235 miles) east of Benghazi, is still under the control of an alliance of Islamists and former rebels called the Derna Mujahideen Shura Council.