Fierce clashes in Libya's eastern city of Benghazi between Islamist militiamen and rival forces loyal to a renegade general have killed 31 fighters on both sides, a security official said Tuesday.

The fighting erupted late on Monday, with forces and fighter jets belonging to Gen. Khalifa Hifter pounding positions of Islamist militias called The Benghazi Revolutionary Shura Council, said the official.

The hours-long clashes concentrated around the city's Benina airport and the militiamen responded with artillery, added the official, speaking on condition of anonymity because he was not authorized to talk to the media.

Hifter's side lost 20 fighters while the militiamen had 11 killed, and 36 fighters in all were wounded, the official said. Several of the wounded were reported to be in critical condition.

Libya is witnessing its worst spasm of violence since former dictator Moammar Gadhafi was toppled and killed in 2011.

The country's divisions are deeply rooted in rivalries between Islamists and non-Islamists, as well as powerful tribal and regional allegiances between groups who quickly filled the power vacuum after Gadhafi's ouster. Successive transitional governments have failed to control the militias.

Fighting in recent months has mostly engulfed the capital, Tripoli, and also Benghazi, the country's second-largest city.

The militias in control of the capital, operating under an umbrella group called the Dawn of Libya, have also taken control of the U.S. embassy compound, a week after they drove out rival militias. A State Department official said the compound "remains secure."

On Tuesday, Libya's official news agency said calm returned to Tripoli, with some banks resuming work and shops and bakeries reopening. Traffic also picked up in the capital and there were long lines outside gas stations. Some families who fled the fighting areas have returned to their homes, the agency said.

In a second chance on Monday, Libya's newly elected parliament asked the country's prime minister who resigned last week, Abdullah al-Thinni, to form a new government. Al-Thinni had said after his resignation that his government had lost control of almost all state institutions and government offices to armed Islamist militias.