"Now some militants are so disillusioned they are looking for any way they can to get out of the fighting," a report by the US military's think-tank, the Centre for Combating Terrorism (CTC) said.
Read more from our special coverage on "ISIS"
The militants are reportedly struggling to keep fighter morale up as the group struggles to deal with territory losses, military pressure, financial problems and poor management.
"This was reflected on a wider level when the Islamic State issued a general amnesty for deserters at the beginning of October 2015. The personnel shortages were also evidenced by an Islamic State document that emerged last year," it said.
Issued in Deir ez-Zor province in eastern Syria, the document indicates that a number of Islamic State members had been seeking false medical reports from doctors in order to avoid front line duty, the Daily Mirror reported.
Much of the terror group's problems with personnel stem from their inability to pay wages.
"The reasons for financial strain on the Islamic State overlap to a degree with the causes of problems of cohesion in the Islamic State's ranks, such as reduced border access to Turkey, tougher border policies, and coalition airstrikes,' the report said.
"These strikes have most recently targeted Islamic State 'cash storage' points and the oil industry."
They have been forced to slash their fighter's wages - and many have quit in disgust at their treatment, the report said.
Earlier this year it was reported that ISIS has been forced to cut the salaries of its murderous fighters by half.
The militants now get the equivalent of just 100 pounds a month because of "exceptional circumstances", according to a document released by Bayt al-Mal, the Treasury Ministry of ISIS.
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