The U.S. State Department is calling on Cuba to immediately release imprisoned American contractor Alan Gross, saying his health has worsened and that he can no longer walk.

State Department spokeswoman Victoria Nuland says Gross, who was arrested more than two-and-a-half years ago, has lost 45 kilograms in detention and is no longer able to walk because of degenerative arthritis.

Gross was arrested in Havana in December 2009 and later sentenced to 15 years in prison on charges of crimes against the state. Gross – who had been working for a private firm contracted by the U.S. Agency for International Development – was accused of distributing Internet equipment and satellite phones to Cuban dissident groups. Gross said he was trying to provide Internet service to members of Cuba's small Jewish community.

Gross has appealed to the Cuban government to return to the United States and visit his seriously ill mother.