ISIL has executed nearly 1,500 people in Syria in the five months since it declared the establishment of a "Caliphate", a monitoring group said Monday.

ISIL has executed nearly 1,500 people in Syria in the five months since it declared the establishment of a "Caliphate", a monitoring group said Monday.

"The opposing UK-based Syrian Observatory for Human Rights has documented the execution of 1,429 people since the ISIL announced its 'Caliphate' in June," the group's director, Rami Abdel Rahman, said.

The majority of ISIL's victims in Syria have been civilians, he said.

"Of the total number of people beheaded or shot dead in mass killings by ISIL, 879 have been civilians, some 700 of them members of the Shaitat tribe."

Another 63 of the dead were members of other terrorist groups or the rival 'jihadist' Al-Nusra Front, which has fought ISIL in the north and east, Abdel Rahman said.

ISIL has executed large numbers of regular troops in recent months.

Many have been beheaded and their bodies put on display in public squares, "in order to strike terror into civilians and into any group that might decide to fight it", Abdel Rahman said.

"Another aim of the ISIL executions is to terrorize the international community, while attracting new 'jihadists' into its ranks," he told AFP.

Syrian activists believe the group is holding hundreds of people hostage.

The Observatory added that Nusra Front captured in Idleb dozens of terrorists of ISIL which in turn arrested a number of Nusra militants in Hamah.