LONDON — About 50 to 100 grams of liquid nerve agent was used in the March 4 attack on the former Russian spy Sergei V. Skripal and his daughter, Yulia, according to the director general of the Organization for the Prohibition of Chemical Weapons.

That quantity — a range from slightly less than a quarter-cup to a half-cup of liquid — is significantly larger than the amount that would be created in a laboratory for research purposes, meaning that it was almost certainly created for use as a weapon, the director general, Ahmet Uzumcu, said in an interview. He added that he did not know the precise amount.

He said he had taken steps to add the nerve agent, one of a series of chemicals created under the code name Novichok, to the list of chemical weapons monitored by the O.P.C.W., a global body created to oversee the elimination of stockpiles after the end of the Cold War.

After that, countries that are signatories to the Chemical Weapons Convention — like Russia, the United States and the United Kingdom — will be required to declare production or stockpiling of Novichok beyond the 5 to 10 grams needed for research purposes, or to develop an antidote, he said.