LONDON — A chemical weapons watchdog amended statements on Friday that its leader had given to The New York Times, in which he estimated that 50 to 100 grams of liquid nerve agent had been used in the March 4 attack on the former Russian spy Sergei V. Skripal and his daughter, Yulia, in Salisbury, England.
A statement issued by the group, the Organization for the Prohibition of Chemical Weapons, said the amount should be measured in milligrams rather than grams.
The group went on to say that it would not be able to estimate how much of the nerve agent was used in Salisbury.
In the interview, Ahmet Uzumcu, the director general of the O.P.C.W., described new measures to monitor production of the nerve agent. Mr. Uzumcu said that countries that are signatories to the Chemical Weapons Convention — like Russia, the United States and the United Kingdom — would be required to declare production or stockpiling of the agent, one of a strain known as novichok. However, he said that countries producing the chemical for research purposes would produce about one-10th the amount used in the attack.
The O.P.C.W., a global body created to oversee the elimination of stockpiles after the end of the Cold War, may add the chemical to the list of weapons it monitors.
Mr. Skripal, a Russian former double agent, and his daughter collapsed on a bench in Salisbury, a city in southern England, several hours after they were exposed to novichok, a nerve agent that Soviet scientists developed for battlefield use against Western troops.
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