Benghazi - Islamic State has executed four people in the central Libyan city of Sirte, including at least one member of a rival group whose body was put on display, according to residents and a video published on social media on Monday.A video released by the militant group showed gunmen shooting a man, dressed in an orange jumpsuit, who was tied to a makeshift cross. His body was left there afterward as a warning to others.The man was identified as an alleged spy for Libya Dawn, an armed group backing a non-recognised government in Tripoli, which has flown air strikes against Islamic State in Sirte.The video's authenticity could not be verified by Reuters.Sirte residents, asking not to be named, said Islamic State had executed a total of four people, which all had been wearing orange jumpsuits. They had no information on their identity.Security vacuumSirte is a stronghold of Islamic State, which has gained a foothold in Libya by exploiting a security vacuum as two rival governments are battling for control four years after the ousting of former leader Muammar Gaddafi.The executions come after Islamic State fighters crushed a revolt by a Salafist Muslim group and armed residents in Sirte earlier this month, trying to break its grip on the city, located some 500km to the east of Tripoli.Islamic State has been trying to retake the eastern city of Derna from where another rival group evicted the group with the help of residents in June. But armed residents repulsed another assault on Sunday, residents said. There was no fighting on Monday.Libya's official government has been based in the east since losing the capital a year ago to Libya Dawn, which set up its own government. Neither administration controls much territory.