Mosul, Iraq - The Islamic State of Iraq and the Levant group (ISIL, also known as ISIS) has been cleared from eastern Mosul for more than a month, but its fighters maintain a significant and sometimes deadly presence in the city.

Some ISIL fighters have shaved their beards, returned to their families and attempted to remain in Mosul unnoticed. Others have formed sleeper cells given the task of launching attacks on Iraqi forces and civilian targets - a return to the bloody tactics long employed by the group's predecessors in Iraq.

Iraq's National Security Service has been working to track down the remaining fighters, launching targeted raids into the city from their base on Mosul's eastern outskirts. The intelligence operatives gather information from a network of local sources, then attempt to apprehend suspected ISIL members.

It is often a race against time. Since Iraqi troops retook eastern Mosul, which is separated from the ISIL-held west by the River Tigris, ISIL fighters have launched a series of attacks on civilian and military targets. In one, a suicide bomber blew himself up among the lunchtime patrons of one of the city's best known popular restaurants, My Fair Lady, which reopened soon after Iraqi forces took the east.

Other blasts have targeted markets and checkpoints. ISIL fighters have also launched a series of attacks with weaponized drones, which have killed and maimed civilians and security officers.

Iraqi security forces are now launching several raids a week into the city and have arrested hundreds of suspects - but there remains much to be done, a task that will get even harder once the west of the city, now the subject of a renewed offensive by government forces, is retaken.