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The feared weapon killed thousands of soldiers in World War One and is now likely to be in the hands of terrorists threatening Britain and the US.

The threat of lone wolf attacks from ISIS fanatics in western cities have been at their highest levels since the regime launched in 2014.

Now fears have been raised the lethal gas could be used by ISIS in heavily populated areas.

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The poisonous gas causes a person's lungs to fill up with fluid and victims eventually stop breathing, leading to death.

The worrying development comes just days after it is believed ISIS smuggled a bomb onto the tragic Russian Metrojet 9268 flight.

The plane exploded over Egypt and came down close to the popular tourist hotspot of Sharm El Sheikh, killing all 224 people on board.

And earlier this week the Kremlin revealed ISIS could be attempting to hijack fighter jets to attack Russian planes.

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Earlier this year, weapons experts found ISIS terror troops using sulfur mustard gas in a Syrian town – where the attack resulted in the death of a baby.

Worryingly, the confidential report by the Organisation for the Prohibition of Chemical Weapons (OPCW) claims ISIS could have made the gas themselves – or are stockpiling it.

The report claims the gas was used in the town of Marea, north of Aleppo, Reuters reports.

It is alleged diplomatic sources say the comical has been used in battles between ISIS and rebels.

Syria surrendered toxic chemicals last year.

A source said "either they (ISIS) gained the ability to make it themselves" or it "may have come form an undeclared stockpile overtaken by ISIS".

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Fears over ISIS capabilities using more advanced weaponry has been raised as Prime Minister David Cameron confirmed it was "more likely than not" the Russian flight last week was brought down by a bomb on-board.

ISIS militants , who claimed responsibility for the disaster last week in a chilling video, were seen handing out sweets to supporters in what appeared to be a celebration following the attack.

Colonel General Viktor Bondarev, head of Russia's Air Force, said ISIS may now be looking to hijack fighter jets in countries neighbouring Syria and use them to attack Russian forces.

Even more worryingly, intelligence suggests the murderous regime has been trying to obtain material to build a radioactive dirty bomb.

All this comes as experts predict the hands on the infamous Doomsday Clock could edge closer to midnight as international tensions rise over Vladimir Putin's position in Syria.

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Just last month a smuggling gang was rumbled selling bomb grade nuclear material to ISIS.

And Australian foreign minister Julie Bishop announced how radioactive materials – only available to governments – had been snatched from research centres and hospitals.

If you thought mustard gas was bad, these are the other weapons you hope ISIS will NEVER get their hands on.

The following weapons are banned in warfare.

NERVE GAS

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Like mustard gas, the nerve gas attack your vital organs and destroy your body by shutting down the nervous system.

They can cause haemorrhaging and painful blisters before death.

BIO-WEAPONS

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ISIS weaponising diseases like smallpox, ebola and anthrax to cause mass death has long been a fear of the west.

The idea of using bio-weapons runs back centuries, with rotting and infected bodies being catapulted into enemy territory to spread disease.

FLAMETHROWERS

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Flamethrowers aren't banned on the war field – unless it is near civilians.

But that's not going to matter to ISIS terrorists, who wouldn't bat an eye lid unleashing hell using a flamethrower on the innocent public.

POISONED BULLETS

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The Strasbourg Agreement of 1675 – the world's oldest arms agreement – outlaws the use of poisoned bullets.

Soldiers would often soak bullets in poisonous or infectious substances due to the inaccuracy of guns.

Poisoned bullets are recognised as chemical and biological weapons in Geneva.

BALLOON BOMBS

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It is against international law to drop bombs from balloons.

But video footage released just last month appears to show ISIS using CONDOM BOMBS to attack Russian fighter jets.

The footage shows ISIS supporters using inflated sheaths tied to explosives, which are then let off into the sky.

DIRTY BOMBS

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International law forbids the use of bombs laced with radioactive material.

Intelligence suggests ISIS are gathering radioactive substances to make their own dirty bombs to attack the west.