EXPLAINER: How to investigate alleged chemical attacks

Ukraine says it is investigating a claim that a poisonous substance was dropped on the besieged city of Mariupol

THE HAGUE, Netherlands -- Ukraine said Tuesday it is investigating a claim that a poisonous substance was dropped on the besieged city of Mariupol. Deputy Defense Minister Hanna Maliar said it was possible that phosphorus munitions — which cause horrendous burns but are not classed as chemical weapons — had been used.

Now the question is how to establish the truth amid the fog of war that has descended over a city still under attack from Russian forces. A clear answer is unlikely to emerge any time soon.

The global chemical weapons watchdog said Tuesday it is “concerned by the recent unconfirmed report of chemical weapons use in Mariupol” and is closely monitoring the situation in Ukraine. Both Russia and Ukraine are among the Organization for the Prohibition of Chemical Weapons’ 193 member states.

The Nobel Peace Prize-winning OPCW says that it “remains ready to assist any State Party upon its request, in case of use or threat of use of chemical weapons.”