WASHINGTON/CARACAS (Reuters) - The United States imposed travel restrictions on Wednesday on some Venezuelan government officials it accuses of rights abuses in a *****down on protests against President Nicolas Maduro this year. Three months of demonstrations led by hard-line opposition and student leaders earlier this year led to violence that killed 43 people in the South American OPEC member nation's worst unrest for a decade. Demonstrators said they were protesting eco**mic hardships and dictatorial government, but Maduro, who replaced the late Hugo Chavez last year, called them a cover for a U.S.-backed "coup" attempt against him.