Once-retired General Khalifa Haftar has been sworn in as army chief by Libya's UN-recognised government based in Tobruk, as his warplanes attacked a rival administration in Tripoli.

As he was sworn in by Prime Minister Abdullah al-Thinni's government on Monday, Haftar pledged to continue battling what he calls "terrorism" until it is eliminated. 

"We will continue fighting terrorism by God’s assistance all over the country until we completely eliminate it so that people can live in peace", Haftar said.

Haftar has been leading a fight against militia groups in Benghazi since last year.

Inside Story: Are more arms the answer for Libya?

With the rival governments engaging in UN-brokered talks to try and form a unity government, Haftar's appointment raises concerns that fighting could escalate.

"Fear is rising that this step could turn into military escalation that might hurt international efforts to end the Libyan crisis," Al Jazeera's Mahmoud Abdelwahed in Tripoli said of his promotion.

Haftar helped Muammar Gaddafi come to power but fell out with him in the 1980s. He joined the 2011 rebellion against Gaddafi and re-emerged on the political stage last year.

The Tripoli government, recognised by Libyan courts, consider Haftar a war criminal.

Warplanes controlled by Haftar continued their attacks on Tripoli on Monday, and his troops remain engaged in a battle for Benghazi.

"Warplanes conducted air strikes this morning on Mitiga airport but there was no damage," Reuters news agency quoted airport spokesman Abdulsalam Buamoud as saying. "Flights were suspended for only an hour ... but now the airport is working normally."

Mohamed al-Hejazi, spokesman for forces loyal to Thinni, said they had attacked the airport "because it's outside state legitimacy ... Weapons and foreign fighters bound for western Libya pass through the airport".

Source: Al Jazeera And Agencies