TRIPOLI, April 6 (Xinhua) -- Libya's eastern-based army on Saturday threatened to attack air bases and fighter jets of the UN-backed government in western Libya, as the army continues its efforts to take over the region.

"The Main Operation Chamber of the Air Force declares the western region a no-fly military zone for any fighter jets or drones belonging to any party," said Ahmad al-Mismari, the army's spokesman, in a statement.

"Any aircraft will be treated as a hostile target, and the air base from which the aircraft took off will also be targeted immediately," al-Mismari added.

A few hours earlier, the government fighter jets attacked the army forces stationed in southern Tripoli, according to a military source.

The army, led by Khalifa Haftar, launched a military operation on Thursday to take over western Libya, particularly the capital Tripoli where the UN-backed government is based.

The army's spokesman revealed on Friday that the army is making significant progress toward the capital city, saying the areas in southern Tripoli are now under the control of the army.

UN Secretary-General Antonio Guterres, who concluded a visit to Libya on Friday, expressed deep concern over the military escalation over Tripoli.

"I still hope it will be possible to avoid a bloody confrontation in and around Tripoli," Guterres told reporters before departing the eastern city of Benghazi.

Libya has been struggling to make a democratic transition amid insecurity and chaos since the fall of former leader Muammar Gaddafi's regime in 2011.