Ammonia is highly toxic and caustic. The chemical should have been removed from the North and residents within 20 KM of the empty tank should have been evacuated for the duration of the conflict. Pure ammonia is used commercially as a refrigerant in industrial freezers. However, any kind of major leak can be instantly fatal if you're close. Exposure at distance and diluted can permanently scar the lungs.
Aden hotel used by Yemen goverment hit by grenade (Reuters)