HAVANA - American students have visited Cuba for years, immersing themselves in Afro-Cuban culture, studying the communist country's revolution and following the trail of Ernest Hemingway.

But those trips are expected to drop dramatically after new U.S. measures aimed at pushing out Cuban leader Fidel Castro and squeezing the island's economy take effect today.

Despite a restrictive U.S. travel ban, American universities with a U.S. government license can bring undergraduate and graduate students for study programs generally lasting from a week to a month. But under the new rules, such trips must be at least 10 weeks long - a requirement critics say will make it impossible for many students to study here.

"Most people can't come for a full semester," said Wayne Smith, an adjunct professor at Johns Hopkins University who brings students to Cuba every year. "Almost everybody comes for a short-term program because that fits into their schedules back home."

"The purpose is to stop educational travel," said Smith, who was chief U.S. diplomat in Cuba 25 years ago and now is a senior fellow at the Washington-based Center for International Policy. "Yet we (the United States) are supposed to be the champions of academic freedoms and the exchange of ideas."

President Kennedy imposed economic sanctions against Cuba in 1963 during the Cold War, with the basic goal of isolating the Cuban government economically and depriving it of U.S. dollars. Forty years later, President Bush has sought more stringent enforcement of provisions that forbid most travel to Cuba.

Original recommendations in a report by the U.S. Commission for Assistance to a Free Cuba complain that most trips by American students are heavily controlled by Cuban state security officials and allow only limited interaction with average Cubans.

The report, completed in May, also denounces travelers and academic institutions that abuse the license by engaging in "disguised tourism." Tourist travel is included in the ban.

A U.S. State Department official in Washington acknowledged that a majority of current trips are short-term, and the new rules would curtail the number of American students visiting the island.