The Danish intelligence agency couldn't confirm, however, whether Abderrahmane, 39, had been killed, The Copenhagen Post reported.
Abderrahmane is known for his time at Guantanamo after Americans captured him in Afghanistan in 2001. In 2011, he sued the Danish government for not preventing is two-year detention in Cuba, the Post reported.
He was one of a growing number of Danes who went to Syria to fight against Bashar Assad's regime, the Post said.
"Slimane was a man who could not stand to see Muslims suppressed, so a few months ago he travelled to Syria to once again perform Jihad and do his duty to fight Allah's enemies," the Facebook group Islamisk Budskab wrote Sunday.