Armed Groups Attack Haftar-Controlled Air Base in Southern Libya

The attack was followed by clashes at the Tamanhint Base located in the vicinity of Sabha, a strategically important city in southern Libya, according to the source.

Haftar's offensive is the latest round of fighting in a years-long struggle for control of Libya following the NATO-backed rebellion that toppled and killed longtime dictator Moamer Kadhafi.

As a result of Haftar's offensive, in April the USA military withdrew a small number of troops from Libya that had been performing diplomatic and counter-terrorism missions against ISIS, citing "increased unrest in Libya". Welcoming the gains by his forces on the ground, GNA head Fayez al-Sarraj called in a statement for humanitarian law to be respected.

Forces loyal to Libya's UN-recognised government have put up strong resistance.

Explosions were heard from the city centre overnight, and some witnesses said there were air strikes.

Pope Francis, delivering his traditional Easter message, called for an end to "conflict and bloodshed" that was killing "defenceless people" in Libya.

"The civilian aviation authority closed the air space of the airport until further notice for the sake of air security", Mitiga airport said on its website, adding that the airport had not been hit.

More than 200 people have been killed since the violence erupted, and more than 900 wounded, the World Health Organization said on Thursday.

Haftar's forces predicted victory within days of beginning its offensive, but the Tripoli government has bogged them down in southern suburbs with help from armed groups from various western Libyan factions.

Reuters was unable to confirm whether an aircraft or unmanned drone was behind the strike which triggered heavy anti aircraft fire.

On Saturday, shelling was louder and more frequent on Saturday than in previous days and audible even in central districts more than 10 km (6 miles) away from the frontline, residents said.

On Thursday, both the United States and Russian Federation said they could not support a U.N. Security Council resolution calling for a ceasefire in Libya at this time.

Prime Minister Fayez al-Serra has condemned the "silence" of his worldwide allies amid the fighting.

Al Jazeera's Mahmoud Abdel Wahed, reporting from Tripoli, said while GNA forces claim Haftar's fighters were on the retreat, the battle would not be easy "because Haftar's forces are supported by regional powers, like Egypt, Saudi Arabia and the United Arab Emirates".

A White House statement said "the two discussed a shared vision for Libya's transition to a stable, democratic political system".