Two of Libya's former armed groups have rejected the government's choice of a new head of the armed forces, Yousef al-Mangoush, which has raised fresh concerns over the security in the country.

In his first international interview, Mangoush told Al Jazeera he understands the challenges he faces in his new role.

The appointment was announced on Tuesday as four fighters were killed in a gun battle between rival armed groups in Tripoli, underlining the interim government's difficulties in controlling the increasingly fractious groups who toppled Muammar Gaddafi.

More than two months after he was captured and killed, real power remains in the hands of the armed groups, who have carved up Libya and its capital into competing fiefdoms, each holding out for the share of power they say they are owed.

Some armed chiefs say they will only cede command of their fighters once an organised military and security apparatus is in place.

Not everyone is convinced with Mangoush's credentials, and convincing the rank and file that he is the man to lead them may be hard. Already brigades from the east of the country have rejected his appointment, preferring someone from their own city to represent them.

Al Jazeera's Imran Khan reports from Tripoli.