The Islamic State has released a new video showing new methods of killing alleged spies and apostates in a bid to show the world that their executions, becoming more and more savage than the last, are in accordance with its twisted interpretation of the Islamic law.

In the seven-minute, Arabic-language video shot in Iraq's Nineveh province, the condemned prisoners said the jihadists had treated them well. Moments later, 15 or 16 of them were shown packed in a car which then exploded like it was attacked by a drone, Fox News reported.

The video, released on Tuesday, also showed the victims being drowned while in a cage. Others were lined up and told to kneel on their knees, with explosives detonating on their necks. Only one was spared, but the ISIS did not explain why he was not executed or who he was.

"While ISIS likes the shock factor, these executions are chosen to show their compliance with Sharia law," said Ryan Mauro, a national security analyst for the Clarion Project. "ISIS is acting upon doctrines that say spies and apostates, those who leave Islam, are to be executed."

"ISIS will argue that these specific methods are justifiable according to Islam as they see it, and that they are not being barbaric just for the sake of it," he said.

"Even its executions are an attempt to portray ISIS as the most legitimate followers of Islam."

The statements by the victims that they were treated well as captives were an attempt by ISIS to repair its damaged image, Mauro said.

"The group includes this because it needs to be seen as humane," Mauro said. "It's hard to believe, but it implies that ISIS recognises it has a negative image and wants to change that while still remaining brutal."

The latest footage also showed that the ISIS is worried about having spies in its ranks, indicating how little the support is for the extremist group.

ISIS' videos for spy executions have a template, according to Veryan Khan, editorial director for US-based Terrorism Research & Analysis Consortium. They begin with a "confession." The victims must be calm before they are killed to show that the ISIS is generous and they are aware that they deserve the punishment. Hostages are well-rehearsed, possibly through mock executions. They do not know that this time they will be killed.

ISIS has been trying to find new ways of executing people to gather attention from the international community, given that their videos have already been so horrific, Khan said.

"Islamic State's propaganda machine is so prolific that it has to keep coming up with new ways to shock the world and impress their fan club. This means that they have to up the ante in order to gain more attention for each new release."

"What is shocking is the style of each execution—it is no longer enough to behead, or to behead 21 men simultaneously, to burn them alive, or shoot them in mass firing squads. They must offer a triptych of executions, each more savage than the last," Khan said.

He described the scene as "haunting and ominous," saying: "This was a message to both potential and current spies; a message to their fan club that the Islamic State is still very much in charge and handling business as usual; a message to the rest of the world that they are not to be trifled with."