An animal rights group said SeaWorld Entertainment Inc, known for its dolphin and killer whale performances, sent an employee on secret missions to infiltrate events organized by the group in an attempt to undermine its work.

For years, People for the Ethical Treatment of Animals (PETA) has campaigned against SeaWorld over its treatment of killer whales. In 2011, the group sued the company for violating the civil rights of a captive orca. The suit was dismissed. Since then, PETA has repeatedly called on SeaWorld to send its captive orcas to seaside sanctuaries.

In a statement on Tuesday, the Norfolk, Virginia-based animal rights group said SeaWorld had become "so upset" that it deployed one of its human relations employees to masquerade as an activist, sending him on undercover missions at protests and organizational meetings in California.

"But, most insidiously, he has repeatedly tried to incite people who object to SeaWorld to act illegally, stating that it's time to 'get a little aggressive,'" PETA said.

In a written statement in response to PETA's allegations, SeaWorld said it was "focused on the safety of our team members, guests and animals, and beyond that we do not comment on our security operations."

SeaWorld has been under growing scrutiny over its killer whale shows, in which the massive marine animals perform tricks and stunts.

In 2013, the company's stock fell 50 per cent after the release of the movie Blackfish, a documentary about a SeaWorld orca and the trainer it killed. SeaWorld was subsequently sued by shareholders and its CEO resigned.