BENGHAZI, Libya -- The death toll from a nighttime twin car bombing near a mosque in a residential area of Libya's eastern city of Benghazi rose to 33 on Wednesday, authorities said.

The Tuesday night attack, which struck the city's Salmani neighbourhood, also left 47 people wounded, local health official Hani Belras Ali said.

No group has yet claimed responsibility for the bombing, but many assumed it was the work of remnants of an Islamic State group faction largely driven out of Libya.

Benghazi police said the attackers timed the second bomb to go off as residents and medics gathered to evacuate the wounded from the first blast, aiming to cause a maximum of casualties.

Khaled Almajdoub said his brother Khalifa, a car mechanic, was killed in the second explosion after he ran out to help the wounded.

"Many people were gathering and more people left the mosque when the second blast happened, and my brother died then," said Almajdoub. "He has four daughters, no political or religious affiliation and was just a small business owner."

The United Nations has condemned the bombings, saying that direct or indiscriminate attacks on civilians are prohibited under international humanitarian law and constitute war crimes. Libya's U.N.-backed government, which is based in the capital, Tripoli, announced three days official mourning.

Neighbouring Egypt, which supports eastern Libya's strongman Khalifa Hifter, also denounced the attack, calling on the international community to "take a firm line on arms smuggling into Libya."

Hifter, a former U.S.-based Libyan opposition member who leads remnants of Libya's National Army in the east, is at odds with the Tripoli government.

Libya fell into chaos following the ouster and killing of longtime dictator Moammar Gadhafi in 2011. Since 2014, the country has been split between rival governments and parliaments based in the western and eastern regions, each backed by different militias and tribes.

Islamic State fighters had established footholds amid the disorder but have been mostly driven out of the main cities.

Benghazi, however, remains a trouble spot, where bombings and attacks still occur. The city has seen fighting between forces loyal to Hifter and Islamist militia opponents.