Mariela Castro, a sexologist who supports gay rights, was granted the visa to attend the annual conference of the Latin American Studies Association on May 23 to 26 in San Francisco.

"We shouldn't be extending an open hand to a regime engaged in the systematic and flagrant denial of basic human rights," Republican presidential hopeful Romney said in a statement.

The Obama administration defended its decision on May 18. "We don't link visa policy in cases like this to our larger political and economic and human rights relationship with countries," said State Department spokeswoman Victoria Nuland.

But Romney insisted the visa for Mariela Castro, who is a government official, was a mistake in handling ties with the Americas' only one-party Communist state.

"While the Cuban regime engages in a fierce crackdown on dissent and continues to unjustly imprison one of our own citizens, Alan Gross, the Obama administration should not be welcoming the daughter of a dictator," Romney said.

"The United States should be standing up for those on the island who are risking their very lives fighting for freedom," he said.

Washington has unsuccessfully pressed for the release of Gross, a State Department subcontractor serving a 15-year sentence in Cuba for espionage.

Washington often rejects visa requests by Cuban officials, but restrictions on professional and artistic exchanges have eased under Obama.

As head of Cuba's National Centre for Sexual Education, Mariela Castro is currently leading a national campaign against homophobia.

Traditionally stigmatised in Cuba, homosexuality was fiercely repressed for many years by the regime, which interned gays in work camps in the 1960s and ostracised them in the 1970s under the rule of Mariela's uncle Fidel Castro.

Raul Castro succeeded his brother Fidel as president in 2006.