By Richard Waddington

GENEVA (Reuters) - A U.N. human rights envoy said Monday dozens of Cuban dissidents were being held in alarming conditions following their imprisonment in a crackdown early last year.

French magistrate Christine Chanet, appointed by the U.N. High Commissioner for Human Rights to probe alleged Cuban abuses, also said her appeals to President Fidel Castro to pardon the dissidents had gone unanswered.

"The personal representative of the High Commissioner has received particularly alarming information about the conditions of detention of these people," Chanet said in her first report on the situation in the Caribbean island state.

According to the reports, prisoners were being frequently transferred from one prison to another, often far from their families, which made visits difficult, Chanet said.

They were being placed in "trying" physical and psychological conditions, whether it be in isolation cells or crammed together with "common criminals," she added.

Cuba triggered a storm of international protest last April when it sentenced some 75 dissidents, some of them over 60 years old, to between six and 28 years in jail on charges of conspiring with the United States to overthrow the Communist-run government.

But Chanet, named to her job in January last year but who has not yet received permission to visit Cuba, also hit out at the United States for its 40-year economic blockade.

"One cannot ignore the disastrous and persistent effects of the embargo...economically and socially, as well as with regard to civil and political rights," Chanet wrote.

The report, which will be presented to the annual session of the Geneva-based U.N. Commission on Human Rights next month, called on Castro's government to grant fundamental freedoms such as that of expression and assembly, and the right to leave the country.

The U.N. commission last year urged Cuba to accept a visit from the envoy in a motion brought by four Latin American countries -- Peru, Nicaragua, Costa Rica and Uruguay.

But Cuba accused them of being "disgusting lackeys" and said the commission would do better to focus its attention on conditions in Guantanamo, the U.S. naval base on the island where suspected al-Qaeda and Taliban prisoners are held.