Islamic State (Isis) militants are facing burgeoning resistance in their de facto capital city Raqqa, with fighters abandoning the terror group and the majority of locals refusing to join, activists working in the city have claimed. Doctors and lawyers have sacrificed their incomes because they no longer want to work on behalf of Isis (Daesh).

Having been witness to horrific public executions, harsh punishments and a brutally oppressive religious police force, Raqqa's citizens are risking their lives to put up "a kind of resistance". This is according to Abdalaziz Al-Hamza – the co-founder of the underground activist group Raqqa Is Being Slaughtered Silently (RIBSS), which secretly operates in the city documenting IS atrocities.

"After two years [under IS rule] there are a million civilians in the city who haven't joined IS," Al-Hamza told Sky News in an exclusive interview. "There are doctors and lawyers without any work and they don't have any money, because the only way to work is to join IS," he added.

Al-Hamza warned, however, that this organic resistance to the terror group is being undermined by airstrikes – particularly those carried out by Russian and Syrian war planes – which are killing dozens of civilians.

Germany-based Al-Hamza accused the Syrian and Russian regimes of callousness, adding that they "don't care if any civilians" are killed by missile strikes. "That will encourage more people to join IS," he added.

RIBSS is considered the most reliable source of information from inside Raqqa – a city where Western intelligence is highly limited. The activist group gathers pictorial, video and written documentation of IS' day-to-day activities – including harrowing evidence of their violence against local citizens.

So potent is the evidence gathered by RIBSS that IS and its supporters have brutally murdered a number of activists – including a handful living in hiding outside Syria. There are currently 17 RIBSS activists working inside Raqqa, however, continuing to gather evidence of the way IS is operating.