Britain says poison used to hospitalize Skripals was in liquid form

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.

In a briefing to media on Tuesday, the Department for Environment, Food and Rural Affairs (DEFRA)said a very small amount of military-grade nerve agent Novichok was used with the substance delivered in a liquid form.

DEFRA said 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 Tuesday that 190 specially trained troops are assisting environmental, health and defence officials with the cleanup in the English city of Salisbury.

British officials say the Skripals were poisoned with a nerve agent made in Russia, which was put on a door handle at Sergei Skripal's house. 

Sergei Skripal remains in hospital, but is no longer in critical condition. Yulia Skripal has been released from Salisbury District Hospital. A British police officer was also treated and released.

The incident has sparked a diplomatic row, with over 20 nations expelling Russian diplomats and officials.

Russia has denied it was behind the attack, and responded by banishing Western diplomats. Most recently, Foreign Affairs Minister Sergei Lavrov has suggested the toxin came from a British or American laboratory.