Turkey has been accused of using chemical weapons on Kurdish forces during its Operation Peace Spring in northern Syria, which Ankara denies.
The Kurdish Red Crescent claims six people have suffered chemical burns, and the US's special envoy for Syria said that US forces had seen evidence of war crimes by Turkish forces during the recent offensive.
US officials said they were looking into reports that Turkey had used prohibited white phosphorus and had demanded an explanation from Turkey's government.
But what is white phosphorous and why is it used in military operations?
Phosphorus comes in the harmless red form and in white, which is highly toxic and reacts violently with oxygen.
The substance was discovered 350 years ago by a German alchemist who was trying to discover the secret of turning lead into gold by boiling urine and burning the residue that was left behind. What was created was a white waxy solid that glowed in the dark and combusts spontaneously, burning with a very bright white flame when exposed to air.
It is, in fact, so volatile that it has to be stored under water or in paraffin to stop it from bursting into flames.
Similar to napalm, white phosphorus is most commonly used to tip artillery shells, with fire that can often be inaccurate, and in grenades. It is used to light up enemy positions, producing an intense heat and thick pillars of smoke, or to firebomb opposing forces.
On contact with a human, it can result in painful chemical burns that may sear flesh to the bone and often prove fatal. It also sticks to clothing or skin and continues to burn unchecked as particles are exposed to air.
White phosphorous is often described by the military as “Whiskey Pete” or “Willy Pete".
While the use of the material is not specifically banned, it is covered by Protocol III of the 1980 Convention on Conventional Weapons, which prohibits use of the substance as an incendiary weapon against military targets located among civilians. The US has not signed it and is not bound by it.
Other examples of white phosphorous use
- In 2002, US troops used the incendiary weapon during the battle for Fallujah in Iraq against insurgents and allegedly civilians. The Pentagon has acknowledged its use. Saddam Hussein had previously used it during the Halabja poison gas attack.
- During the Gaza War in 2008 and 2009, the Israeli army were accused of the widespread use of white phosphorus in crowded civilian residential areas, with Amnesty International's fact-finding team saying they found "indisputable evidence".
- Israel also said it had used phosphorous bombs in Lebanon during a conflict between the two nations in 2006.
- In 2009, the US military said it had documented 44 incidents of Afghan insurgents using or possessing white phosphorus ammunition.
- Both the US-led coalition and the Syrian regime, along with their allies Russia, have been accused of using the substance in Syria.