CARACAS, March 10 (Reuters) - Venezuelans woke up to a fourth day of an unprecedented nationwide blackout on Sunday, leaving residents concerned about the impacts of the lack of electricity on the South American country's health, communications and transport systems.

Socialist President Nicolas Maduro - who is facing a challenge to his rule by the leader of the opposition-led congress, Juan Guaido - has blamed the blackout on an act of "sabotage" by the United States at the Guri hydroelectric dam, but experts say it is the outcome of years of underinvestment.