The World Health Organization took the unprecedented step of warning that the world is probably on the verge of a pandemic as new cases of swine flu mounted. The first death was reported in the United States and the virus appeared to arrive just outside the nation's capital. The agency raised the alert level for the second time in three days, elevating it to one notch below a full-scale pandemic, after concluding that the virus was causing sustained outbreaks in the United States and Mexico. The heightened alert is intended to prompt every nation to activate an emergency response plan, to spur pharmaceutical companies to increase production of antiviral drugs and help speed development of a vaccine, and to prod bankers to help poor countries afford measures to fight the virus, officials said.