UN envoy Ghassan Salame last week warned of "the increasing frequency of attacks" on Mitiga airport

Libya's rival forces have taken their battle for Tripoli to the skies as fighting stalls on the ground, alarming UN envoy Ghassan Salame who has stepped up calls for a truce.

More than 1,000 people have been killed since commander Khalifa Haftar launched an offensive on April 4 to take the capital, but there have been no major breakthroughs on the battlefield.

Fighters loyal to the UN-recognised Government of National Accord (GNA) have kept Haftar's self-styled Libyan National Army (LNA) at bay on the southern outskirts of the city.

"The inability of both one camp and the other to make military progress... is pushing them to rethink their military tactics," said Khaled al-Montasser, a professor at Tripoli University.

Montasser said that the two sides were now targeting each other's "rear bases, supply centres and troop concentrations".

With the front lines largely static, both sides are resorting to strikes using warplanes and drones supplied by their foreign supporters to span the large distances between populated centres.

The LNA said it carried out strikes over the weekend on 10 targets, including a military academy in Misrata, some 200 kilometres (120 miles) east of the capital.

Five doctors were killed in the air raids, according to the GNA.

The raids came after air strikes Friday on Haftar's strategic air base of Al-Jufra, 650 kilometres south of Tripoli. Its fall would severely degrade the LNA assault.

- Action plan -

Salame has responded to the escalation with calls for a ceasefire on the Muslim religious holiday of Eid al-Adha which falls around August 10.

"The decision to stop the war cannot be postponed indefinitely. I therefore submit the following three-part" action plan, he said Monday in an address to the UN Security Council from Tripoli.

The UN envoy proposed that a ceasefire be accompanied by confidence-building measures such as an exchange of prisoners and the remains of fallen fighters.

"Second... I request a high-level meeting of concerned countries to cement the cessation of hostilities, work together to enforce strict implementation of the arms embargo," which in theory has been in place since Libya's 2011 revolt that toppled Moamer Kadhafi, he said.

The final step would be "a Libyan meeting of leading and influential personalities from all over the country to agree on comprehensive elements for the way forward".

Salame stressed that "it should be abundantly clear to all that the risks of either an open-ended low-intensity conflict or a full escalation to outright war on the shores of the southern Mediterranean are equally unacceptable".

The main obstacle, however, remained that "the parties still believe they can achieve their objectives through military means".

The fighting since April has left nearly 1,100 people dead and wounded more than 5,750, according to the World Health Organization.

More than 100,000 civilians have fled their homes.