International chemical weapons inspectors have announced they are gathering information following accusations that burning white phosphorus was used by Turkish forces against children in Syria earlier this week.

The Organisation for the Prohibition of Chemical Weapons (OPCW) said on Friday morning that “it was aware of the situation and is collecting information with regard to possible use of chemical weapons”.

The Kurdish Red Crescent said in a statement that six patients, both civilian and military, were in hospital in Hasakah with burns from “unknown weapons” and it was working to evaluate what had been used.

It said it could not confirm the use of chemical weapons and was “working together with our international partners to investigate this subject”.

A British chemical weapons expert sent photographs from a former colleague of a child with a badly burned torso in a frontline hospital said he thought it was likely they showed chemical burns.

Quick guide White phosphorus Show Hide What is white phosphorus? White phosphorus, known as WP, is a chemical that burns fiercely in contact with air, producing thick white acrid smoke and a white light that can be useful for illumination. The high temperature at which it burns also makes it an effective incendiary device in war. Is it legal? International humanitarian law allows the use of white phosphorus in munitions for making smoke to mask troop movements and for illumination purposes. Its use as an incendiary weapon is generally understood to be forbidden in all circumstances where there is a risk to civilians, ie against military targets located amid concentrations of civilians. Its use against military targets, while permitted, is discouraged if there are other means to render enemy combatants hors de combat because of the suffering caused by phosphorus burns. It is a chemical, but is it a chemical weapon? While white phosphorus has toxic chemical properties, it is not generally used to poison or asphyxiate, the common aim of chemical weapons. For this reason, rules governing its use tend to fall under the wider provision of humanitarian law rather than specific prohibitions on chemical weapons. Peter Beaumont

Hamish de Bretton-Gordon, a former commander of the UK’s chemical, biological, radiological and nuclear regiment, said: “The most likely culprit is white phosphorus. It is a horrific weapon, and has been used repeatedly during the Syrian civil war; unfortunately its use has become increasingly normalised.”

White phosphorus is routinely held by militaries around the world and is used legally in combat as a smokescreen in daytime and as an incendiary to light up an area at night. But it is illegal to use it against civilians, because it causes serious and exceptionally painful burns on contact with skin.

On 7 October the US president Donald Trump abruptly announced that it was time to pull American troops from north-east Syria, removing the buffer stopping Turkey attacking the Kurdish-led Syrian Democratic Forces, who had spearheaded the fight against Islamic State.

An incursion ensued, and despite a US-brokered ceasefire announced on Thursday, fighting was continuing in northern Syria on Friday. Turkey regards the Kurdish fighters as proxies for the Kurdistan Workers’ party, which has waged a 35-year insurgency against the Turkish state.

Kurdish officials accused Turkey of using “unconventional weapons” in the conflict on Thursday – a few hours before the ceasefire was announced – and called international inspectors to examine the injured.

A member of the Rojava diplomacy office on Friday said it had documented a number of suspicious cases and restated calls for international organisations to investigate since it lacked capacity on its own.

Turkey said the accusations were false. Hulusi Akar, the defence minister, said: “It is a fact known by everyone that there are no chemical weapons in the inventory of the Turkish armed forces.”

The incident is alleged to have taken place in or around the border town of Ras al-Ayn but there are few confirmed details about what happened and whether civilians were deliberately targeted. White phosphorus is most commonly used to tip artillery shells, whose fire can often be inaccurate.

The OPCW said it had “not yet determined the credibility of these allegations” and that the chemical weapons inspectorate would continue to monitor the situation.

Under the terms of the ceasefire, the SDF would pull back from Turkey’s proposed 20-mile (32km) deep “safe zone” on its border.

The incursion and the ceasefire have been widely criticised, including by the EU council president Donald Tusk, who described the latter on Friday as “a demand of capitulation of the Kurds”.

In a rare joint statement the chairs of the foreign affairs committees in the UK, France, Germany, the EU and the US House of Representatives condemned the incursion as a military aggression and a violation of international law. The joint statement also said Trump’s decision to abandon the Syrian Kurds was likely to lead to a resurgence of Islamic terrorism, “marking another landmark in the change of American foreign policy in the Near and Middle East”.

Recep Tayyip Erdoğan, the Turkish president, is due to meet his Russian counterpart, Vladimir Putin, in Sochi on Tuesday, where it is expected more concrete talks on the size of Turkey’s planned buffer zone will take place.

Erdoğan is also due to meet Boris Johnson, Emmanuel Macron and Angela Merkel in London in the coming weeks.

• This article was amended on 29 October 2019 to clarify that the OPCW is not a UN agency.