Cuba's Supreme Court has upheld the conviction of jailed U.S. contractor Alan Gross, who was sentenced to 15 years in prison in March for crimes against the communist state.

The court on Friday rejected an appeal from Gross, who was arrested by Cuban authorities in December 2009 for bringing communications equipment into the country.

The United States condemned Friday's ruling and urged Cuban authorities to immediately release Gross. State Department spokesman Mark Toner called Gross a dedicated international development worker who has devoted his life to helping people in more than 50 countries. Toner said Gross was in Cuba to help the Cuban people connect with the rest of the world.

At the time of his arrest, Gross was working for a private firm that contracted with the U.S. Agency for International Development. He was accused of distributing Internet equipment and satellite phones to Cuban dissident groups. Gross said he was trying to improve Internet access for the island's small Jewish community, and that his actions were not intended to be a threat against Cuba's government.

The White House has described the sentencing of Gross as “another injustice.”

His family has appealed on humanitarian grounds for the 62-year-old contractor to be returned home.

The United States and Cuba do not have formal diplomatic relations, only interests sections that are technically part of the Swiss embassies in each other's capitals.