LNA spokesman Ahmad Mismari said on Tuesday that the army had captured 21 "terrorists" in Derna, after army forces advanced south of the city.
"The armed forces carried out a pre-planned combat mission today, where the enemy positions in Duhr al-Hamr, al-Tila, and Matrouba areas were controlled. The enemy suffered heavy losses of individuals and vehicles," Mismari said in a statement.
The spokesman also said the arrested terrorists descent from six foreign nationalities, and confirmed that the army is "proceeding with a steady and orderly pace towards the liberation of Darna from terrorism."
The army is launching a military operation to take over Derna from the extremists who control the coastal city.
Since 2015, Derna has been surrounded by the LNA, demanding that the Shura Council of the Mujahideen in Derna, a coalition of Islamist militias, leave. The army accuses the armed group of being loyal to Al-Qaeda.
Earlier in April, Haftar announced that his forces would take over Derna "as demanded by the people."