Malaysian police have said the substance used in the killing of Kim Jong-nam was a VX nerve agent.

The substance – also known as ethyl N-2-Diisopropylaminoethyl Methylphosphonothiolate – is classified by the UN as a weapon of mass destruction.

Kim Jong-nam died on 13 February after a woman had sprayed chemicals on his face at Kuala Lumpur airport.

CNN reports that the Centers for Disease Control and Prevention describes VX as the most potent of all nerve agents and one that would be lethal on contact with the skin unless it was immediately washed off. CDC says that the only known use of VX is in chemical warfare.

VX is a clear, colorless liquid with the consistency of engine oil. According to the Council on Foreign Relations, a fraction of a drop, absorbed through the skin, is enough fatally to disrupt the nervous system. VX was developed in the 1950s by a British chemist researching pesticides, but was later found to be too toxic to be used safely.

Saddam Hussein used VX during the Iran-Iraq War, and the Assad regime in Syria still has small quantities of VX at its disposal, even though Syria was supposed to relinquish all of its chemical weapons.

North Korea, which is not a member of the Chemical Weapons Convention (CWC), is in possession of a large stockpiles of chemical weapons — between 2,500 and 5,000 metric tons, with Sarin and VXmaking up the bulk of the arsenal.