WASHINGTON - The Obama administration on Thursday called on Cuba to immediately release jailed American contractor Alan Gross, saying his health has worsened and he can no longer walk.State Department spokeswoman Victoria Nuland said the administration also is urging authorities in Havana to turn over information about his condition to his family so they can assess his health and whether he is receiving adequate medical care."He has lost more than 100 pounds since his arrest. He suffers from severe degenerative arthritis and is no longer able to walk in his cell. He has other health problems that cause pain and require treatment," Nuland said."We call on the government of Cuba to release Alan Gross immediately and allow him to return to his family," Nuland said.Gross, 63, was arrested in Havana in December 2009 and sentenced to 15 years in prison for setting up Internet networks under a semi-covert US program aimed at promoting political change.Havana saw his work as part of longstanding US efforts to subvert Cuba's Communist society established after Fidel Castro's 1959 revolution.US-Cuba relations have warmed somewhat since President Barack Obama took office, but that progress has stalled as a result of Gross' imprisonment.Gross has asked Cuban authorities for permission to visit his gravely ill mother, an appeal Nuland said the United States supports.