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The United States is about to begin destroying its largest remaining stockpile of chemical-laden artillery shells, marking a milestone in the global campaign to eradicate a debilitating weapon that still creeps into modern wars, The Associated Press reported.

The Pueblo Chemical Depot in southern Colorado plans to start neutralizing 2,600 tons of aging mustard agent in March as the U.S. moves toward complying with a 1997 treaty banning all chemical weapons.

"The start of Pueblo is an enormous step forward to a world free of chemical weapons," said Paul Walker, who has tracked chemical warfare for more than 20 years, first as a U.S. House of Representatives staffer and currently with Green Cross International, which advocates on issues of security, poverty and the environment.

The work starts less than a year after chlorine gas killed 13 people in Syria in April 2014. International inspectors concluded last month that the gas had been used as a weapon.

Before the chlorine attack, 1,400 people were killed in a 2013 nerve gas attack in Syria, the U.S. said.

Pueblo has about 780,000 shells containing mustard agent, which can maim or kill, blistering skin, scarring eyes and inflaming airways. Mustard agent is a thick liquid, not a gas as commonly believed. It's colorless and almost odorless but got its name because impurities made early versions smell like mustard.