Isis militants are using chlorine gas in roadside bombs attacks, as a means of spreading terror among Iraqi forces, officials said.

A video shows bomb disposal team detonating a device, from which orange smoke billows, reports the BBC.

The chemical agent is used in small quantities in the devices, but is deadly under prolonged exposure.

Haider Taher from the Iraq Bomb Disposal Team said that the troops had defused dozens of the devices on operations to seize back territory from the militant group.

"They have resorted to this new method," he told the BBC. "They're putting chlorine inside these homemade roadside bombs, which is toxic for those that inhale it."

He said his team accidentally detonated one of the devices near Tikrit recently, where Iraqi forces and Shia militias are engaged in an operation to retake the city.

"Our throats were blocked, we couldn't breath. My ears felt enormous pressure... we were lucky a military ambulance was there to treat us," Taher said.

Chlorine gas was used in World War I, first at the battle of Ypres in 1915, and though not as deadly as specialised nerve agents, it causes burns, vascular and respiratory failure.

Under sustained exposure, it can kill in minutes.

There have been a number of insubstantiated claims that Isis has used chemical weapons.

In October 2014, when it was alleged that poison gas had been used against Kurdish fighters in defending Kobane in northern Syria.

Isis is believed to have seized old but potent chemical weapons from army bases in Iraq when the militant group seized swaths of Iraq in June 2014.