Chemical weapon experts have initiated an investigation of a possible chemical attack by Turkish forces on America’s Kurdish allies in northern Syria, The Guardian and other publications reported Friday.

The Kurdish Red Crescent has reported that at least six Kurds, some of them civilians, including children, are being treated for burns suffered after an attack in or around the border town of Ras al Ain with what appeared to be white phosphorus.

A representative of the intergovernmental Organization for the Prohibition of Chemical Weapons, which works with the United Nations, told The Guardian on Friday that officials were “aware of the situation” and “collecting information with regard to possible use of chemical weapons.” The credibility of the attack has “not yet been determined,” according to the spokesperson.

Foreign Policy magazine reported that Turkish forces or their supporters appeared to be using munitions loaded with white phosphorus.

Photos provided to Foreign Policy — and confirmed by a senior U.S. administration official — show children from Ras al Ain with severe chemical burns.

Hamish de Bretton-Gordon, a former commander of Britain’s chemical, biological, radiological and nuclear regiment, told The Times of London that the burns look like they were caused by white phosphorus. “White phosphorus is a horrific weapon, which can be delivered by aircraft or artillery,” he told the newspaper. “It reacts to the moisture in the skin in a way that intensifies its burning, so that water cannot put it out.”

Military forces around the world use the chemical legally in combat as a smokescreen or to illuminate an area at night. But it’s illegal to use it against civilians.

Despite an agreement for a “pause” in fighting negotiated by Vice President Mike Pence and touted Thursday by President Donald Trump, fighting was continuing in Ras al Ain, where 38 war wounded being treated in a hospital cannot be easily evacuated, noted Foreign Policy.

Turkey denied accusations it used chemical weapons against civilians. “It is a fact known by everyone that there are no chemical weapons in the inventory of the Turkish armed forces,” Defense Minister Hulusi Akar told reporters.

In an interesting twist, the OPCW announced it launched its investigation into a possible chemical attack a day after it revealed a $39,000 donation from Turkey to the organization.