WASHINGTON, Dec. 2 -- The Obama administration on Monday reiterated its call for Cuba to release U.S. contractor Alan Gross, denouncing as "unjustified" his continued imprisonment.

The fresh appeal from the State Department came as Gross, 64, will begin his fifth year on Tuesday of what the agency called " unjustified imprisonment" in Cuba.

"It is gravely disappointing, especially in light of its professed goal of providing Cubans with internet access, that the Cuban government has not allowed Mr. Gross to return to his family, where he belongs," the department said in a statement.

Cuban authorities arrested Gross at a Havana airport on Dec. 3, 2009, accusing him of providing satellite communication equipment for anti-government groups. A Cuban court handed down a 15-year prison sentence to him in March 2011 for spying and working to topple down the Cuban government.

"We reiterate our call on the Cuban government, echoing foreign leaders and even Cuba's allies, to release Alan Gross immediately and unconditionally," the State Department said.

The case had added a new source of friction between Washington and Havana, which have not had full diplomatic relations since January 1961.