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The Islamic State has added a unique, deadly weapon to its cache in its bid to expand the Islamic Caliphate in the Middle East – 'scorpion bombs'.

The ISIS militants are now launching bombs that contain live scorpions into towns and villages of Iraq, letting the poisonous creatures crawl out and unleash fear among residents.

The new tactic was highlighted by British military expert Hamish de Bretton-Gordon, who returned from Iraq last week.

Gordon described the improvised weapons as meant for 'psychological impact' and to incite fear.

"Scorpions are robust – even if they are launched a couple of miles, when the canister breaks thousands are flung out and start crawling all around. Some scorpions are very poisonous but the main thing is creating fear," Gordon told The Mirror.

Gordon said that the scorpion bombs were about 2 feet long but were not meant for mass casualties. Scorpions have been used as weapons earlier by Iraqis about 2,000 years ago to defend against Roman invasion.

"It's madness. IS have improvised devices to launch them. They promote the fact that they are doing it and it creates panic," he said.

Gordon had earlier warned that the Islamic State had the ability to make 'dirty bombs', radiological dispersal device (RDD) that combines conventional explosives, such as dynamite, with radioactive material.

He had said that the ISIS could access stockpiles of mustard gas and sarin gas, that are used as chemical weapons.