* Cuba begins biggest war games in five years
* Says must be ready for U.S. invasion despite recent thaw
* Maneuvers include preparation for social unrest
HAVANA, Nov 26 (Reuters) - Cuba began its biggest military maneuvers in five years on Thursday, saying they were needed to prepare for a possible invasion by the United States.
Despite a thaw in U.S.-Cuba relations and assurances last week by President Barack Obama that the United States has no intention of invading the island 90 miles (145 km) from Florida, Cuba's state-run press quoted military leaders as saying there "exists a real possibility of a military aggression against Cuba."
The war games, which are being called "Bastion 2009," also will get the military ready to deal with social unrest the United States may try to foment in this time of economic crisis in Cuba, ahead of an invasion, they said.
Cuban television showed images of tanks firing their guns as they rolled through the countryside, artillery batteries blasting away, camouflaged troops digging trenches and shooting bazookas, attack helicopters and fighter jets buzzing through the sky and rescue teams tending wounded combatants.
It was not clear if the images came from Thursday's maneuvers or from file footage of previous activities, nor were the sites of the war games disclosed.
Tanks and anti-aircraft guns were seen on trains outside of Havana on Thursday being prepared for transport to an unknown destination.
In the evening news broadcast, President Raul Castro was shown urging Cubans to fight until they have vanquished the enemy.
"The objective is to never surrender, to never stop fighting," he said in a meeting with military leaders.
"Fight and fight until we exhaust the enemy and defeat it," said Castro, who was Cuba's defense minister before succeeding his ailing brother Fidel Castro as president last year.
The maneuvers, which end on Saturday and will involve 100,000 soldiers and reservists, are taking place at a time when relations between the United States and Cuba have warmed under Obama after five decades of hostility.
He has slightly eased the 47-year-old U.S. trade embargo against the communist-led island and initiated talks on migration and postal service, but based further progress on Cuba releasing political prisoners and improving rights.
President Castro has said Cuba is open to better relations, but will make no unilateral concessions to the United States.
In a written response to questions from Cuban dissident blogger Yoani Sanchez last week, Obama said, "The United States has no intention of using military force in Cuba."
NECESSITY OF FIRST ORDER
But Cuban military leaders have insisted in state-run press that Bastion 2009 is "a necessity of the first order in the current political-military situation that characterizes the confrontation between Cuba and the empire (the United States)."
They appeared to signal disgruntlement with Obama, whose election brought high hopes of change on the island, saying the embargo goes on and he has not removed Cuba from the United States' list of "terrorist" countries.
History is also a factor. Cuba, fresh from the 1959 revolution that put Fidel Castro in power, fended off a U.S.-backed invasion by Cuban exiles at the Bay of Pigs in 1961 and has remained on high alert for another ever since.
At the height of the Cold War, Cuba entered into an alliance with the Soviet Union and received military support until the former superpower collapsed in 1991.
The alliance almost brought the world to the brink of nuclear war in 1962 when the Soviets placed nuclear missiles on the island, prompting a showdown with the United States that became known as the Cuban missile crisis.
The tense confrontation ended peacefully when the Soviets withdrew the missiles in exchange for a U.S. pledge to never invade Cuba and, it was later revealed, pull its own missiles from Turkey.
Additional reporting by Rosa Tania Valdes and Esteban Israel; editing by Mohammad Zargham
Our Standards: The Thomson Reuters Trust Principles.