Singapore animal-rights activists on Wednesday repeated calls for a casino resort to release dolphins from a marine life park after four of them died in captivity.

The Society for the Prevention of Cruelty to Animals (SPCA) said in a statement that a bottlenose dolphin named Sharmila died on May 11 at Resorts World Sentosa (RWS), which originally acquired 27 dolphins for its marine park.

"Subjecting these wild dolphins to a forced lifestyle in captivity, tamed against their will is... in essence unabashed animal exploitation," Corinne Fong, SPCA's executive director, said in a statement.

"We urge RWS to release the remaining 23 wild-caught dolphins and end the exploitation of these animals," Fong said.

Holding the dolphins in captivity and "training them to become something they are not" will not lead to constructive education of the public on marine life, she added.

Fong's comments Wednesday echo similar calls by other Singaporeans groups including the Animal Concerns Research and Education Society (ACRES).

RWS, owned by Malaysian business group Genting, acquired 27 dolphins from the Solomon Islands in the South Pacific between 2008 and 2009 and sent them to the Philippines to be trained while the marine park in Singapore was being built.

In a statement to AFP, the resort disputed SPCA's claims that all four dolphins died while in captivity at its marine life park.

RWS said two had died in 2010 from a water-borne bacterial infection while at the Malaysian resort island of Langkawi, while another died in November 2012 due to an infection aboard a flight to Singapore.

"There will always be divergent views about animals in human care and in zoological environment," RWS said.

"Our viewpoint is that well-run zoological facilities provide strong and inspiring messages to visitors and can make a tangible difference to animal conservation," it added.

RWS, which also hosts Universal Studios Singapore, attracted more than 6.7 million visitors in 2013, according to the resort's latest annual report. It posted total revenues of Sg$2.85 billion ($2.27 billion) last year, down from Sg$2.95 billion in 2012.