The US is about to begin destroying its largest remaining stockpile of chemical weapons, marking a milestone in the global campaign to eradicate a debilitating weapon.

The Pueblo Chemical Depot in southern Colorado plans to start neutralising 2,600 tons of ageing mustard agent in March.

It is part of the long-running US effort to comply with a 1997 treaty banning all chemical weapons.

Chemicals still creep into modern warfare. Less than a year ago, in April 2014, chlorine gas killed 13 people in Syria. No group claimed responsibility for the attack.

Mustard agent can blister the skin, cause scarring on eyes and inflame airways. Pueblo has about 780,000 shells containing the chemical.

Blue Grass Army Depot in Kentucky also has a stockpile of chemical weapons. Destruction at the facility will start in the next year or two.