Castro's letter which was read out by Venezuelan president Hugo Chavez in the gathering of leftist leader in Havana, said that US was plotting to topple Chavez and other leftist government by supporting the the rightist movement.
"The empire is mobilizing behind rightist forces in Latin America to strike Venezuela and, in doing so, strike other leftist states," Castro noted.
He said that Obama was trying to cover up his real intention behind his kindly smile.
Castro who initially favored Obama's victory became increasingly critical of the US president.
Recently, he condemned Obama for accepting the Nobel Peace Prize at the time when he was preparing for deployment of more troops in Afghanistan.
Fidel Castro was the president of Cuba until ill-health forced him to step down.
He was succeeded by his brother Raul Castro in 2008