London, Apr 17:  British officials say the nerve agent used to poison former Russian spy Sergei Skripal and his daughter was delivered in liquid form, and it will take months to remove all traces of the toxin. The Environment Department says nine sites need "specialist cleaning," including a restaurant visited by the Skripals on March 4, the day they were found unconscious on a park bench.

The department said today that 190 specially trained troops are assisting environmental, health and defense officials with the clean-up in the English city of Salisbury. British officials say the Skripals were poisoned with a military-grade nerve agent made in Russia, which was put on a door handle at Sergei Skripal's house. Yulia Skripal has been released from Salisbury District Hospital, where her father continues to be treated.

Nerve gas or nerve agents are toxic chemicals, widely used in warfare for targeting large number of people. The gas tends affect the nervous system and impairs the communication between the brain and the nerves, disrupting importantly bodily functions. Although it is called a gas, nerve agents are liquid at room temperature and produce a vapour that is inhaled by victims.

It was originally intended to be used as insecticides, but its high level of toxicity and durability made it an efficient weapon for military use. Nerve agents are known to be one of the deadliest weapons for warfare since they are cheap and easy to produce and can cause mass-scale destruction. This is probably why it is preferred even by developing nations.