Forces loyal to Libya's unity government said Saturday they have launched a counter-attack against strongman Khalifa Haftar's fighters just south of the capital Tripoli.

"We have launched a new phase of attack. Orders were given early this morning to advance and gain ground," said Mustafa al-Mejii, a spokesman for the Government of National Accord's forces.

Haftar launched an offensive on April 4 against Tripoli, where the UN-recognised GNA is based.

The military commander backs a rival administration based in eastern Libya that refuses to recognise the authority of the Tripoli government.

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Sustained rocket and shellfire could be heard in several districts of the capital on Saturday, after several days of less intense fighting and stalemate on the ground.

"After coordination, our forces have launched vast attack operations," particularly on the Wadi Rabie, Sawani and Ain Zara fronts in the southern suburbs of the capital, said Rida Issa, another spokesman.

The GNA's main military spokesman Colonel Mohamad Gnounou said seven air strikes had been carried out against military positions held by Haftar's self-styled Libyan National Army (LNA).

Gnounou said strikes took place south of Gharian, 100 kilometres (60 miles) southwest of the capital, and against an airbase at Al-Wotya, 50 kilometres further southwest.

The LNA, for its part, said its fighters were "taking control of several new positions on the frontlines in Tripoli".

"Our forces are progressing as the GNA militias retreat on all fronts," the LNA said on its Facebook page.

At least 213 people have been killed and more than 1,000 wounded since the violence erupted, according to the World Health Organization, while the International Organization for Migrations says over 25,000 people have been displaced.