Nearly 5,000 Syrians have been murdered in captivity by the Islamic State group

Almost 5,000 Syrians have been murdered in captivity by the Islamic State group over the past four years - including 2,750 civilians - the Syrian Observatory for Human Rights said on Saturday.

The extremist group, which has lost significant swathes territory in the last year, marked four years of its self-declared "state" in Syria on 29 September.

In the last month alone, the SOHR has recorded 30 "executions" in IS' Syrian territories, including in Raqqa, Deir az-Zour, and the Syrian desert.

Of those executed, 11 were Syrian citizens, three were IS members, seven were fighters loyal to the Syrian regime, while nine were from the US-backed Syrian Democratic Forces .

A total of 4,954 people - including regime fighters, civilians, rebels and IS members - have been murdered by the IS group in Syria during the group's rule in Syria.

The group's methods of execution have included beheading, stoning, throwing off high places and burning.

Those slain were reportedly accused of collaborating with rebels, US-backed Syrian Democratic Forces or the regime.

Prisoners were also murdered for adultery, desertion and a number of other alleged crimes.

While not entirely defeated, the IS group has suffered huge losses in the last year in Syria with over 90 percent of its urban stronghold Raqqa captured by the Syrian Democratic Forces. Many of its fighters having retreated to the extremists' eastern stronghold of Deir az-Zour.  

In recent days, Russian-backed Syrian regime forces were reported to have all but encircled Islamic State militants in the city, however the group recently launched a counter-offensive.

Many of the group's fighters have also been forced to retreat from Iraq into Syria, following defeat in their former bastions of Mosul and Tal Afar this year.

In an attempt to rally its fighters, IS on Thursday released an audio recording purportedly from its leader and 'caliph', Abu Bakr al-Baghdadi.

The recording was the first audio message said to be of Baghdadi since November 2016, when he defiantly urged his supporters to fight against the Iraqi forces that went on to capture Mosul and its US backers.