"They are criminal gangs, ideological groups, thugs and outlaws," al-Serraj told Euronews in exclusive interview
Migrants and refugees caught in the Mediterranean are brought back to Libya and kept in detention centres where they are particularly vulnerable.
Fighters loyal to the Government of National Unity are trying to defend their city from a two week long attack by the eastern based Libyan National Army.
The UN-backed Libyan government says at least 21 have died in clashes between government forces and the rebel Libyan National Army
Eastern Libyan commander Khalifa Haftar ordered his troops to march on the capital Tripoli as the UN tries to push for a peace deal.