US theme park SeaWorld has suspended an employee who allegedly infiltrated the animal rights group PETA -- but activists denounced the move as cosmetic and said they could unmask more spies.

People for the Ethical Treatment of Animals (PETA) has long clashed with the Miami-based marine tourist attraction, which it accuses of cruelty to animals notably over its treatment of killer whales.

Earlier this week the group alleged that SeaWorld planted an undercover agent in its ranks, who it said took part in PETA rallies and incited activists to commit violence.

In response SeaWorld said: "The allegations made... against a SeaWorld employee are very concerning. These allegations, if true, are not consistent with the values of the SeaWorld organization and will not be tolerated.

"We have placed the employee in question on paid administrative leave pending the findings of the investigation," it added in a statement.

But PETA said the move was not enough. "'Suspending' your own agents is an old trick, which usually comes with a backroom deal of compensation and a promise to bring them back when things die down," it said.

And it said it believes more undercover agents can be exposed. "We do not believe that SeaWorld has limited its espionage efforts to (the suspended agent's) activities.

"It has hired protesters to attend SeaWorld rallies, and PETA is currently looking at two more men who we believe were SeaWorld agents hired to infiltrate PETA as 'volunteers,' and the list may grow," it said.

SeaWorld admitted last year that its income had fallen in part because of campaigns by activists against the use of the killer whales in its shows.

An acclaimed 2013 documentary "Blackfish" also spotlighted the impact of captivity on orcas in examining an attack in which an orca killed a trainer.

The company is building much larger tanks to hold its orcas, to be completed by 2018.