Islamic State has executed a Syrian it accused of belonging to a militant cell that ambushed its fighters and blew up its vehicles in the town of al-Mayadin near the border with Iraq, a monitoring group that tracks the conflict said on Thursday.Islamic State is fighting both the Syrian army and rival insurgents along the edges of the territory it controls. But there has also been a spike in guerrilla-style attacks by groups of Syrians against Islamic State fighters inside their strongholds in Raqqa and Deir al-Zor provinces.The Syrian Observatory for Human Rights, which uses a network of sources across the country to report on the war, said the man was beheaded and his body crucified in a public park in the town with his head left displayed for three days.He was accused of "setting up a cell to fight Islamic State" by staging ambushes and detonating vehicles of fighters of the militant group, it said.The Observatory, which is based in Britain, said four other men, one a student, were executed in Deir al-Zor for alleged ties with the Syrian authorities."One was arrested for smoking a cigarette. Only later did Islamic State accuse him of being an informer for the regime," Observatory head Rami Abdulrahman said. Islamic State bans smoking in areas under its control in line with its strict interpretation of Islam.Syria's state news agency SANA said Islamic State executed three civilians and displayed their bodies in al-Mayadin. It did not say what their crimes were.Small groups of Syrians who are hunting down Islamic State fighters say it is part of a guerrilla campaign that has emerged as a response to the group's growing brutality.