TRIPOLI, June 14 (Xinhua) -- The UN Support Mission in Libya (UNSMIL) on Thursday expressed alarm by alleged human rights violation in Libya's eastern city of Darna.

A video footage emerged online showing a group of army soldiers in Darna beating and killing two people in civilian clothes suspected of being detained prisoners.

"UNSMIL is alarmed by the allegations of serious human rights abuse in Darna, including pillage, destruction of property, arbitrary detention and a video of summary executions. If proven, these could constitute war crimes," the mission said in a statement.

Army Spokesman Ahmad Mismari later on Wednesday said that the army commander, General Khalifa Haftar, has called for investigation into the alleged illegal execution of the two people, and ordered the army to commit to the instructions regarding the treatment of detained prisoners.

The mission also demand the investigation be thorough and independent. It added that "perpetrators must be held accountable, in line with the International Humanitarian Law and Human Rights Law."

"Fighters who have surrendered, laid down their weapons, or hors de combat must be treated humanely. Civilians must be protected," it added.

The army is launching a military operation to take over Darna from the militants who control the coastal city.

The army has besieged Darna since 2015, demanding that the armed group of the Shura Council (Consultative Council) of the Mujahideen, a coalition of Islamist militias, leave. The army accused the group of being loyal to al-Qaida.

Mismari announced on Sunday that the army has taken over more than 90 percent of Darna, expecting full control of the city in the next few days.