Dolphins have gone missing from a recreational facility in the Japanese town of Taiji, which is at the centre of an international controversy over its annual hunts.

Officials at the DolphinBase centre reported to local police that nets at two of the facility's nine seaside pools had been vertically slashed, allowing four bottlenose dolphins to get out.

According to the facility's official blog, three out of four returned on their own accord.

Japan's NHK newsreported that two of them remain missing.

The escaped dolphins were spotted swimming outside their pool at the facility, where visitors can touch and swim with them. On the blog, the centre wrote it was "very angry" at the "malicious act" which could lead to the dolphins dying.

"They think that once out of their pen, dolphins will swim far away but that is not true. Dolphins will not stray far and they will not leave their group," it said.

Local say they do not know who's behind the incident.

A police spokesperson told the BBC that an investigation was ongoing.

Activist group Ric O’Barry’s Dolphin Project condemned the episode, saying that while they are against keeping dolphins in captivity "we do not condone illegal behaviour."

"Our Cove Monitors operate fully within Japanese law, documenting Taiji’s dolphin drive hunts for the Japanese people and the rest of the world to see – including the horrific capture methods and continued slaughters."

Taiji's annual dolphin hunt, which takes place from Sept. 1 to March 1, caused an international outcry after it became the subject of an Academy Award-winning documentary in 2009: The Cove.

Local fishermen herd hundreds of dolphins and pilot whales into a small bay using a method called "drive fishing."

Many film stars spoke out against the hunt, including Game of Thrones actress Maisie Williams.