HAVANA (AP) — Cuba's armed forces launched three days of intense military exercises across the island Thursday, a mobilization that state-controlled media says is designed to guard against an American invasion.

Americans focused on a U.S. military assault more likely are thinking about how President Barack Obama will pursue war in Afghanistan — not Cuba. But the siege mentality of the Cold War hasn't faded on the island, where the communist government continues to warn about imperialist aggression and the menace from the north.

The exercises, which run through Saturday, are the first since President Raul Castro took over from his brother Fidel in February 2008 — and since relations between Havana and Washington began to thaw somewhat under Obama.

The U.S. leader has loosened financial and travel restrictions on Cuba. The two countries have begun negotiations on restarting direct mail service, and there is talk of future cooperation on counter-narcotics and disaster relief, among other things.

More than the specifics, officials on both sides speak of a new tone between Havana and Washington that has made further progress a possibility.

But the rhetoric connected with Thursday's mobilization — dubbed "Bastion 2009" — displayed none of that new warmth.

Radio Rebelde said the attack was aimed at "confronting a possible aggression by North American imperialism." The state-run newspaper Granma called the mobilization the largest and most important in more than five years.

The exact number of troops involved are not known, but past exercises have involved hundreds of thousands of people — both uniformed and civilian.

"The current political-military situation that characterizes the confrontation between Cuba and the U.S. government has made these strategic exercises a necessity of the first order," said an article on the Radio Rebelde Web site. All Cuban media is tightly controlled by the government.

Analysts say Cuba is more concerned with sending a message to those who would seek to destabilize the country than with an actual military assault.

"I don't think it is so much that they expect an invasion or anything like it," said Hal Klepak, a Cuba military expert and professor emeritus at the Royal Military College of Canada. "I think what they worry about is disorder in Cuba of any kind that would lead to blood in the streets."

Such a show of force is particularly important, Klepak said, given the open question of who would succeed Fidel and Raul Castro, aged 83 and 78, and because of Cuba's current economic difficulties.

But he said a fear of outside agitation is not far-fetched given America's long history of intervention in Cuba and the strong anti-Castro feelings of some in the exile community.

In 1961, U.S.-backed Cuban exiles launched the ill-fated Bay of Pigs invasion against Fidel Castro's fledgling communist government. A year later, the world came to the brink of nuclear Armageddon after the Soviet Union stationed missiles on the island, and the United States insisted they be removed. Washington has maintained an economic embargo on Cuba for 47 years.

Obama has said he would like to see Cuba's government enact social, political and economic reforms. But he has categorically ruled out a military invasion, most recently in written comments made last week to Cuban dissident blogger Yoani Sanchez.

The military exercises began in the 1980s and have taken place sporadically since then, most recently in 2004. They were meant to be held in 2008, but had to be canceled because of the need to use the armed forces to help rebuild after several large hurricanes hit the island, causing billions of dollars in damage.