The U.S. government said the health of Alan Gross, an American contractor behind bars in Cuba since 2009, has deteriorated to the point that he is unable to walk, calling on the island's government to hand over the results of his latest medical examination to his family.

"We are extremely concerned about Alan Gross' health. His health has seriously deteriorated during his incarceration. He has lost more than 100 pounds since his arrest," State Department spokeswoman Victoria Nuland said Thursday.

"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," she added.

The spokeswoman urged Cuban authorities to hand over the results of his latest medical exam to his relatives so they can assess whether he is receiving proper care.

Gross' attorney in the United States, Peter J. Kahn, confirmed Thursday that he demanded in a letter sent to the Cuban Interests Section in Washington that Havana provide Gross' family the results of a medical exam his client underwent six weeks ago.

Kahn, who also stated that Gross' health has deteriorated significantly, said the Cuban government has not replied.

The attorney said in addition to the severe weight loss and arthritis Gross also has developed a mass behind his right shoulder blade.

Although Cuban doctors have told Gross that mass is a hematoma, his family is concerned and asked for a medical report, as well as MRIs and other scans taken of his hip so doctors in the United States can review them.

Nuland also cited reports from family members indicating the contractor is increasingly depressed over the Cuban government's refusal to allow him to visit his 90-year-old, terminally ill mother.

"We call on the government of Cuba to release Alan Gross immediately and allow him to return to his family, putting an end to this injustice that began more than two years ago," Nuland said.

Now 63, Gross was arrested in Havana on Dec. 3, 2009, in possession of satellite communications equipment he said he was planning to distribute among Cuba's Jewish community.

Havana says he was illegally aiding dissidents and inciting subversion on the Communist-ruled island. Last August, Cuba's highest court upheld the 15-year jail sentence imposed on Gross five months earlier.

Gross was in Cuba as an employee of a Maryland firm contracted by the U.S. Agency for International Development.

Washington has dismissed suggestions that it trade five Cuban intelligence agents convicted of espionage in Florida more than a decade ago for the contractor.

Havana, which has publicly hinted it would be prepared to free Gross outright in exchange for the return of the "Cuban Five," acknowledges the men were intelligence agents but says they were spying on Miami's Cuban exile community, not the U.S. government.

Nuland said last month there was no "equivalence" between the situations because "on the one hand, you have convicted spies in the United States and on the other hand you have an assistance worker who should never have been locked up in the first place."

In a phone interview with CNN aired in May, Gross said he was being held at a "secured hospital building" with three people to a room as opposed to a "typical Cuban jail."

He said he "didn't really see any sunlight for the first year-and-a-half or so" behind bars, his "food was infested with insects" and he was not allowed to read anything.

But added that "eventually, after the conviction and after the Supreme Court upheld the conviction, things improved as far as my physical comforts were concerned."

The U.S. State Department has repeatedly asked Cuba to heed Gross's request to visit his dying mother as a reciprocal measure after Washington allowed one of the Cuban Five - Rene Gonzalez, on probation in Florida after serving 13 years for espionage - to travel to the island in late March for a brief visit with his terminally ill brother. EFE