US animal rights activist Ric O'Barry, detained for nearly three weeks after being denied entry to Japan, has been deported from the country, his supporters said Friday.

The star of the Oscar-winning documentary "The Cove" about dolphin killing in a town of Taiji, "has been placed on an airplane and deported from Japan, where he has been incarcerated for 19 days," said a statement from Dolphin Project, a conservation group he heads.

O'Barry had been fighting for entry into Japan after immigration authorities refused to let him in last month and he was held at the airport until his departure.

Japanese immigration officials cited his past trip to an area where did not report to authorities when visiting Japan last year on a tourist visa, it said.

"It is ironic that they are deporting me to keep me quiet, when they themselves have brought more attention to the dolphin slaughter than 'The Cove' movie," O'Barry said in the statement.

"It breaks my heart to be deported from a country I have grown to love."

"The Cove", which won an Academy Award in 2010, drew worldwide attention to the annual dolphin hunt in the small Japanese town of Taiji.

The 76-year-old O'Barry lost more than 10 kilogrammes (22 pounds) and suffered a minor chest problem during his stay at a "jail-like facility of the Immigration Bureau at Narita Airport, Tokyo," Dolphin Project lawyer Takashi Takano said in the same statement.

"Mr O'Barry's visits to 'The Cove' in Taiji and his reports on dolphin hunting should be considered a legitimate tourist activity," Takano said.

"To those who believe Japan is an open and democratic country, it must be shocking to realise this kind of experience can happen here and now."

Japanese immigration officials have declined to comment on the incident, as they do not discuss the status of individual cases, and could not be reached after office hours Friday.

The website did not provide details on what time or to where O'Barry, a US citizen, was deported.

O'Barry has bumped up against the Japanese legal system before.

In September he was arrested near Taiji on the eve of the start of the controversial six-month dolphin hunt for allegedly failing to carry his passport after being stopped by police.

He was released the following night.

In recent years, Japanese police have dispatched more officers to Taiji during the hunt in anticipation of possible clashes between locals and activists from conservation group Sea Shepherd.

O'Barry first found fame in the 1960s for catching and training five dolphins for the well-known television series "Flipper", but he has recently fought against keeping the mammals in captivity.