Cuba's President Fidel Castro temporarily signed over his presidential powers to his younger brother Raul after undergoing intestinal surgery.

The Cuban government made the announcement Monday night in a live televised statement signed by Fidel, who turns 80 on Aug. 13, and read out by his secretary Carlos Valenciaga.

In the letter, Castro said he had suffered gastrointestinal bleeding, which he blamed on the stress of recent public appearances in Argentina and Cuba.

This ""had provoked in me a sharp intestinal crisis with sustained bleeding that obligated me to undergo a complicated surgical procedure," read the statement.

Due to the operation, Castro said he was forced to rest for several weeks.

Castro said he was giving his 75-year-old brother power because Cuba was "under threat from the U.S. government."

The following posts have been temporarily delegated:

• First secretary of the ruling Communist Party
• Commander in chief of the military
• President of the executive council of state

Castro postponed his 80th birthday celebrations from Aug. 13 to Dec. 2, the date of the 50th anniversary of Cuba's Revolutionary Armed Forces.

Raul Castro made no public statement.

In Havana, people were initially stunned by news of the transfer of power, but most put on brave faces for foreign reporters. However, one Castro opponent said he must be very sick to step down even temporarily.

"It's almost the same as death," Martha Beatriz Roque told The Associated Press. "No one knows if he'll even be alive Dec. 2 when he's supposed to celebrate his birthday."

Meanwhile, in the Cuban areas of Miami, Florida, Castro's foes celebrated in the streets over this evidence of his possible mortality.

White House spokesman Peter Watkins said: "We are monitoring the situation. We can't speculate on Castro's health, but we continue to work for the day of Cuba's freedom."

Castro's reign

Fidel Castro has led Cuba since the Jan. 1, 1959 revolution, overthrowing then-President Fulgencio Batista.

Raul, the government's defence minister, is his designated successor. In a bid to stop an eventual succession by Raul, the U.S. has tightened sanctions on the country and increased funding for Cuba's rebel movement.

On April 16, 1961, Castro said Cuba's revolution was a socialist one. The next day, Cuba stymied the Bay of Pigs invasion, carried out by exiles but sponsored by the United States.

In late October 1962, the Cuban missile crisis -- which began with U.S. President John F. Kennedy's announcement that the Soviet Union had installed nuclear missiles in Cuba -- brought the world as close to the brink of all-out nuclear war as it has ever been.

Cuba is one of only five communist governments remaining in the world. The other four are China, Vietnam, Laos and North Korea.

The former Soviet Union had supported Cuba economically, but that ended in 1991 with the USSR's dissolution. While the United States had imposed an economic blockade early on in Castro's rule, that blockade was tightened in 1992 and 1996.

While the country had erased illiteracy and provided universal health care, those gains were offset by a loss of political rights.

Castro characterized U.S. demands for multiparty elections and an open economy as de facto regime change.

However, Canada and Cuba maintained full diplomatic and economic relations, although Canada was largely unsuccessful in provoking political change.

In 2001, speculation over Castro's health was sparked when he fainted during a speech made in the hot Cuban sun.

In October 2004, he stumbled after a speech, resulting in a fractured knee and arm.

Castro chuckled at a 2005 rumour he had Parkinson's Disease.

"They have tried to kill me off so many times,'' Castro said in a November speech.

However, Castro said if his health deteriorated significantly, he would ask the Communist party to find someone to take over.

With files from The Associated Press