A day of fighting in the weeks-long battle for control of the international airport in the capital Tripoli has killed at least 22 people, Libya's interim government says.

In a statement released early on Sunday, it said "heavily armed groups" shelled "civilian targets" endangering thousands of citizens and leaving hundreds of families displaced.

The statement came a day after Libya's newly elected parliament, or House of Representatives, held its first official session in a heavily guarded hotel in Tobruk, a coastal city, as armed factions turned the two biggest cities, Tripoli and Benghazi, into battlefields.

Near the capital Tripoli, oil depots burned after being hit by shells fired by various rival groups.

"Eight fuel tanks are now on fire, including the ones we had put out only days ago", Mohamed al-Harrari, National Oil Cooperation spokesman, told Al Jazeera.

"The firefighters can't work. It's too dangerous."

The smoke rising from the tanks, which contain benzene and household cooking gas, filled the sky of southeast Tripoli, more than 20km east of the depot.

The 22 people killed on Saturday were the latest casualties in fighting that has claimed more than 200 lives in recent weeks.

Grip of violence

Libya is in the grip of its worst violence since the 2011 uprising that toppled long-time ruler Muammar Gaddafi and thousands are fleeing the violence into neighbouring Tunisia.

With its national army still being formed, the country has struggled to control heavily armed factions that have entrenched themselves as de facto power brokers in the transition since Gaddafi's overthrow.

Armed Islamist groups from the coastal city of Misrata have led the assault on the airport, seeking to seize it from the groups from the mountain town of Zintan.

The fighters are mainly former rebels who toppled Gaddafi with the aid of NATO air strikes.

While the parliamentarians gathered in Tobruk on Saturday, a block of southern Tripoli - located at one of the front lines between warring factions - was on fire after being hit.

Nearby streets were littered with shell casings from machine-gun fire where Zintan fighters had defended their positions, the Reuters news agency reported.

The emergency session of the new parliament, elected in June, aims to form a new government that many Libyans hope will be a step to ending the crisis.

"Our homeland is burning," Abu Bakar Baira, interim head of parliament, said. "We have to work fast, to meet the demands of the people and save them from this disaster."

The 200-member parliament is expected to elect its new president on Monday.

Western governments, which have mostly evacuated their diplomats after two weeks of fighting, hope the elected body can create space for negotiations in the face of worsening violence.

Britain on Saturday became the latest Western government to announce it would close its embassy, fearing being caught in the crossfire.