At least 33 people have been killed in a double car bombing in Libya's eastern city of Benghazi, according to officials.

Tuesday's first bomb went off outside Salmani neighbourhood's Bait Radwan mosque, frequented by the fighters of Brigade 210 from eastern Libyan security forces, the Libya Observer reported.

The second explosion occurred minutes later close to the first blast site after security and health officials arrived.

Ahmed al-Fituri, chief of a special investigation unit attached to the general command of east Libyan security forces, was killed in the first explosion, Libya Observer cited military sources as saying.

It was not immediately clear who was responsible.

The twin explosions shattered the relative calm that had recently returned to the country's second city, scene of more than three years of warfare from 2014 until late last year.

Benghazi conflict

Forces loyal to eastern-based commander Khalifa Haftar prevailed in Benghazi's conflict after protracted urban battles against its opponents that left parts of the port city in ruins.

There were several bombings during the latter stages of the conflict targeting figures linked to Haftar's Libyan National Army (LNA).

The LNA claimed victory in Benghazi in July, but sporadic clashes dragged on until last month, when it took control of its rivals' final holdout.

It has imposed strict military controls on Benghazi and other parts of eastern Libya.

The fighting in Benghazi was part of a broader conflict that developed in Libya after former ruler Muammar Gaddafi was removed from power and killed in a NATO-backed uprising in 2011.

The North African nation has had competing governments aligned with rival military factions based in Tripoli and the east since 2014.

The eastern government, which is aligned with Haftar's LNA and opposes an internationally backed government in the capital, declared three days of mourning after Tuesday's attack.

The United Nations condemned the attack on social media, saying that direct or indiscriminate attacks on civilians are prohibited under international humanitarian law and constitute war crimes.