The Islamic State (ISIS) has introduced a new weapon that has been described as "unique, strange, and terrible."

The weapon is an undetectable car bomb that the terrorist group used in Baghdad on July 3—its deadliest attack yet that killed 292 Iraqis, the BBC reports.

"Daesh [known in the Western world as ISIS] used, for the first time, a new tactic which helped it to move undetected through checkpoints," a Western security source told the BBC.

"We've never seen it before, and it's very worrying."

The way the ISIS placed the explosives in the van and the amount of chemicals they put together to make a VBIED—vehicle-borne improvised explosive device—were unique, according to the report.

"It's really difficult to make," an explosives expert said.

The bomb-makers are believed to have taken a formula "available on the Internet," and then adjusted the quantities of the chemicals and explosives to reduce the risk of detection and increase the bomb's impact at the same time.

"We are used to big fires but the chemicals in this bomb were used for the first time in Iraq," said Brigadier General Kadhim Bashir Saleh of the Civil Defence Force.

"It was unique, strange, and terrible."

The terrorists parked the van on a narrow street and detonated the bomb inside just after midnight shortly before the Eid Festival when the shops were packed with people.

Witnesses said the heat created by the first blast was "as hot as the surface of the sun."

Surprisingly, the huge explosion left no gaping crater, and its shock wave did not destroy the nearest buildings.

But the blast set off secondary fires which resulted in the inferno that trapped many people inside the buildings housing the shops. The buildings were later found to be in violation of safety regulations because of lack of fire escapes and other measures.

Perfume products that fuelled the flames, cheap styrofoam walls and bad wiring also contributed to the large fatality count.

One of the firemen who rushed to the scene said he had "never seen anything like it" in describing the blazing orange fireball that engulfed the entire street.

"We were ready to jump into the fire to save people. We did everything we could but this was an overwhelming attack," Chief Sergeant Habib Dewan said.