Islamic State jihadists are now allegedly using IEDs containing noxious chlorine gas in an attempt to both poison and scare pro-government forces in Iraq.

Footage captured by an Iraqi bomb disposal squad and published by the BBC shows thick clouds of orange smoke rising from roadside bombs detonated in controlled explosions.

The squad said that they had detonated "dozens" of similar devices during the Iraqi offensive against the militants. However, the weapons only contained small amounts of the chemical and used in this way they are unlikely to prove lethal. The chlorine bombs are likely to be employed as a psychological weapon instead, designed to provoke panic and fear.

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

Taher added that he and his team had accidentally detonated one such weapon outside Tikrit over a month earlier and quickly felt the results. "Our throats were blocked, we couldn't breath. My ears felt enormous pressure," he said. Other details on the use of the gas remain scarce.

Chlorine, commonly used to treat water and clean clothing, was first used in battle a century ago during a 1915 World War I battle in Ypres, Belgium. It is considered a "dual-use" chemical under the 1997 Chemical Weapons Convention (CWC) and is nowhere near as lethal as dedicated nerve agents.

Its deployment as a weapon is strictly prohibited for CWC signatories, however, and exposure can cause burns and, if inhaled, respiratory failure and vascular abnormalities. High-level exposure can kill in minutes.

There have been persistent reports of Islamic State using weaponized chlorine against Iraqi troops since October 2014, although concrete proof has not been forthcoming thus far. Syrian-Kurdish medical staff and politicians also alleged in October that the jihadists had used an unknown chemical agent during their offensive on the border enclave of Kobane.

Meanwhile, in Iraq, pro-government forces are continuing a major offensive to retake Tikrit from the Islamic State and have reportedly now entered the city itself. Clashes continued on Thursday and the operation's military commander told the Associated Press that second phase of the assault — an attempt to reach the city center — would begin later in the day.

Tikrit, the birthplace of deceased Iraqi dictator Saddam Hussein, is around 100 miles north of Baghdad and was seized by Islamic State in June 2014 as the group swept across the north of the country in a shock offensive. Government forces have made several unsuccessful attempts to retake the city since, but this is the largest yet and comes as the Islamic State has suffered defeats elsewhere in recent months, including the nearby oil refinery town of Baiji.

A 30,000-strong mixed force made up of Iraqi government troops and security forces alongside Shia militias and Sunni tribes launched the assault on March 2. Commanders from regional Shia powerhouse Iran have also taken a leading role in the operation, a development described by the US military as "positive," but which has also raised concerns about the risk of increased sectarian tensions.

So far the attackers have advanced on towns to the north and south of Tikrit as well as from the east, partly encircling the town, but Islamic State fighters were understood to remain in control of the city itself. Progress has been slow as the vastly outnumbered extremist militants resorted to guerrilla tactics, such as booby-traps and snipers.

If Tikrit is retaken, it would be both a propaganda and tactical victory for Iraqi forces and help pave the way for an assault on the country's second city of Mosul, which was also seized by Islamic State in June.