WASHINGTON -- Cuban President Raul Castro’s announcement over the weekend that he’ll step down in 2018 after the five-year term he just began ends starts the countdown for U.S. officials contemplating a thaw in relations with the island nation. But analysts caution that so far the regime’s reforms amount to window dressing.

By law, the United States is restricted from normalizing relations with Cuba as long as the island is ruled by the Castro brothers: ailing revolutionary leader Fidel, 86, and his brother Raul, 81.

Raul Castro said Sunday that not only would he step aside in 2018, he also would propose term limits and age caps for future presidents, the latest in a series of moves that are hailed by some Cuba observers as steps toward reform but dismissed by others as disingenuous.

But those are hardly the kinds of breakthrough reforms that State Department and independent analysts say will be needed to improve U.S.-Cuba relations, which froze after the Cuban revolution of 1959 that saw Fidel Castro align himself with the communist bloc and the United States impose a trade embargo that 54 years later remains in place.

“Each side is making small, subtle moves, but since it’s a glacier, it’s not going to melt overnight,” said Alex Crowther, a former U.S. Army colonel and Cuba specialist whose published commentaries on bilateral relations include a 2009 essay calling for an end to the embargo.

Analysts of U.S.-Cuban relations said that the latest moves are primarily self-serving for the regime, allowing the two elderly brothers to handpick an acceptable successor before they’re too infirm to administer the country.

Raul Castro’s anointing of Communist Party stalwart Miguel Diaz-Canel, 52, as the favored successor was the most important takeaway from the president’s speech, several analysts agreed.

“It doesn’t mean he’s being chosen to succeed Raul, but it does mean they’re leaving the gerontocracy and opening up the aperture to younger leaders,” Crowther said.

Diaz-Canel is “an impressive career politician,” said Jorge Dominguez, a Cuban American professor of Mexican and Latin American politics and economics at Harvard University. He moved through the Communist Party ranks, serving as a provincial first secretary, minister of higher education, a member of the party’s political bureau and one of the Castro’s gaggle of vice presidents.

“In those roles, he has a wider array of responsibilities that have positioned him well for the eventual succession,” Dominguez said. “He has also been traveling abroad with Raul to add foreign experience to what had been principally a domestic-policy resume.”

When Castro elevated Diaz-Canel to first vice president and set a date for his own stepping aside, for the first time there was an expiration date for Castro rule of Cuba.

“It is true that other would-be successors appeared from time to time, but none was anointed, and none had a formal designation as the successor,” Dominguez said. “Sure, there will be political fights in the future. Theirs is a political party, after all, and politicians will jockey for power and position. But Diaz-Canel is now the frontrunner.”

The Castro brothers know by now that such moves also play well in the United States, where they just got a public relations boost with the remarks of a U.S. senator who led a delegation to Cuba this month to seek the release of Alan Gross, an American imprisoned on the island for illegally importing communications equipment while on a USAID-funded democracy-building program.