Image copyright AP Image caption Three dolphins have since returned but one remains missing

A pod of dolphins has escaped from a recreational facility in the Japanese town of Taiji, known for its annual controversial hunts.

Staffers from the DolphinBase centre discovered nets dividing the pools from the ocean had been slashed, allowing four dolphins to get out.

An official blog said the dolphins had "stayed close to their pen" and that three returned on their own accord.

Local police say they do not yet know who was behind the incident.

The bottlenose dolphins were estimated to be between three to five years old. They were being trained in the seaside pens having been held there for more than six months.

The park would not tell the BBC whether they were wild or captive-bred.

The escaped pod were spotted swimming outside their pool at the facility, where visitors can interact and swim with the animals, on Wednesday.

The centre wrote on its official blog that it was "furious" someone with "no expert knowledge had callously exposed the dolphins to danger".

"We are enraged by this heinous act which can easily lead to the dolphins dying," the statement said.

"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."

The last dolphin remains outside the pen but nearby. The centre said it was "scared" of the new entrance and did not know how to get back in.

Image copyright AP Image caption Taiji's dolphin hunt has carried on for decades despite international outcries

A police spokesperson told the BBC that an investigation was ongoing and the perpetrators remain unknown.

A similar incident was carried out in 2010 by a Dutch group called Blackfish, who left a knife behind.

Activist group Ric O'Barry's Dolphin Project condemned the actions of those involved in a statement.

"While we are against keeping dolphins in captivity, we do not condone illegal behavior," it said.

The group which monitors activity in the bay added: "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."

Japan's annual controversy

Taking place between September to March, Taiji's annual dolphin hunt first gained global attention when it became the subject of 2009 Oscar-winning documentary film The Cove.

Hundreds of dolphins and pilot whales are herded by local fishermen into a small bay, using a method called "drive fishing".

They are then either killed with knives for meat or sold to aquariums.

Image copyright AP Image caption Game of Thrones star Maisie Williams lent her support to activists condemning the hunt

The annual catching and slaughtering of dolphins and whales in Japan has courted international criticism.

In 2015, Japanese aquariums voted to stop acquiring dolphins caught during the controversial hunt following a suspension from the World Association of Zoos and Aquariums (Waza) because of the way dolphins are caught.

British and US ambassadors as well as celebrities have also joined environmentalist groups in condemning the "cruel" practice.