Castro, 80, underwent intestinal surgery last July, and handed over control of the communist Caribbean country - for the first time during his 47 years as leader - to his brother and potential successor Raul, 75. Speculation has since spread over whether Castro will return to power.
"Fidel grows stronger each day and is gaining weight," Jorge Martinez said at a press conference at RIA Novosti. "He is actively participating in solving key issues on the daily agenda."
But Martinez did not specify when Castro will fully return to power in Cuba saying only that Fidel is a respected person in his country with big authority.
The ambassador also said that Moscow will host May 29 a regular meeting of Russia-Cuba Intergovernmental Commission on Trade, Economic, Scientific, and Technical Cooperation, which will be attended by Cuban Government Minister Ricardo Cabrisas.
Cooperation between Russia and Cuba has been developing in all spheres Martinez said, adding that "the perspectives for further development in our relations have become significantly wider."
The former Soviet Union for decades provided strong economic backing to the Latin American state, helping it survive more than 40 years of sanctions imposed by the United States, its Cold War enemy, which sought to topple the Castro regime.
But the support was halted in the 1990s, when the Soviet empire collapsed and Moscow had to deal with an acute political and economic crisis at home.
Prime Minister Mikhail Fradkov visited Cuba last September, and reaffirmed that Russia would continue to press for the removal of U.S. sanctions.
Fradkov said then Russia would provide Cuba with a 10-year $355-mln loan to be used to finance the delivery of Russian goods and services in 2006-2008.
The money will reportedly be used to modernize Cuba's energy sector and transport system, rebuild water conservation facilities and railroads, and design and deliver air navigation systems.