The United Nations Libya mission said it was alarmed by reports of "brutal and outrageous summary executions" in the eastern city of Benghazi on Wednesday after pictures emerged appearing to show at least nine people were shot dead at the site of a twin car bombing.
The UN Libya mission, UNSMIL, suggested in a tweet that the gunman was Mahmoud al-Werfalli, a special forces commander wanted by the International Criminal Court (ICC) for allegedly carrying out a number of similar killings.
"(The UN) demands the handing over of Mahmoud al-Werfalli immediately to (the ICC) as it documented at least 5 similar cases, in 2017 alone, carried out or ordered by al-Werfalli," UNSMIL said on its Twitter account.
"Those responsible for committing or ordering summary executions are criminally liable under international law."
Reuters could not independently confirm the gunman's identity.
The photos, which were posted on social media and in local media, appear to show executions in front of Benghazi's Bayaat al-Radwan mosque, where a twin car bombing on Tuesday left at least 35 people dead.
One of the pictures shows a gunman dressed in military camouflage, pointing a weapon at the head of the first of a row of blindfolded men kneeling in blue jumpsuits in front of damaged mosque gates.
Another photo shows all but three of the people slumped forward on the road as the gunman makes his way along the line. It was unclear who the men were.
Al-Werfalli is from an elite unit attached to Khalifa Haftar's Libyan National Army (LNA), which controls Benghazi and battled Islamists and other opponents in the city until late last year.
After the ICC said it was seeking al-Werfalli's arrest in August, the LNA announced that it was investigating him and had detained him, though his whereabouts were unclear. The special forces dismissed the ICC warrant.
A spokesman for the LNA could not immediately be reached for comment on the pictures.
The pictures were posted as UN Libya envoy Ghassan Salame visited Benghazi on Wednesday, meeting Haftar at a military compound near the city and expressing condemnation of the car bombing.
Haftar, a divisive figure in Libya, has increasingly been courted by the international community as he has gained power on the ground. He is considered a likely candidate in elections that the U.N. has said it hopes can be organised by the end of 2018.