The "window to prevent mass death is rapidly closing, and for many it's already too late," one senior humanitarian official told NBC News.

"In August and September, there are probably still going to be ... large numbers of deaths because children have already passed the tipping point," Mark Brauner, who volunteered…

"In August and September, there are probably still going to be ... large numbers of deaths because children have already passed the tipping point," Mark Brauner, who volunteered…