Russia has released a group of eight captive whales back into the wild after an international outcry over several months from wildlife activists and celebrities.

The Russian Research Institute for Fisheries and Oceanology said the whales, two killer whales and six beluga whales – the first of almost 100 caught in the country’s far east – were released into the Sea of Okhotsk this week.

However, Greenpeace Russia and wildlife activities claim the mission to free the whales, transported in small containers across 1800 kilometres, was not transparent, was rushed, and lacked appropriate rehabilitation before release.

They had been held in icy waters in a so-called ‘whale jail’ in a Russian bay for months.

The institute issued a statement saying the whales, tagged with tracking devices, were closely monitored and were in good health.

“Vets conducted all the necessary tests before they were freed, and the results showed the animals are in good health,” the institute said.

Up to 100 whales were caught last year and were intended for the Chinese market, to be sold to aquariums and marine parks.

Celebrities including Leonardo DiCaprio petitioned for their release at the time, tweeting in March a petition from change.org describing the plight of the whales.

“Please sign this petition and join me in speaking out against the inhumane capture of orcas and belugas in Russia,” DiCaprio wrote.

Please sign this petition and join me in speaking out against the inhumane capture of orcas and belugas in Russia. https://t.co/OED9zJ9fnz

— Leonardo DiCaprio (@LeoDiCaprio) February 26, 2019

Russia has begun the process of returning whales from "whale jail" back to the area where they were captured. Jean-Michel Cousteau and Whale Sanctuary Project urge government to include international specialists and Russian observers.
https://t.co/TWjA2St1lr

— Whale Sanctuary Project (@Whale_Sanctuary) June 26, 2019

Greenpeace issued a statement saying the animals were released without appropriate rehabilitation, with two orcas reportedly swimming along the shoreline for hours before finally heading out to sea.

It said the sudden release seriously raised the “risk of trauma or death for the animals” and “the whole process took place in secrecy” without international experts and scientists on hand to assist.

Jean-Michel Cousteau, son of the late French marine expert Jacques said he had heard concerns from the public about a “lack of transparency” in the operation.

President of the Ocean Futures Society, Jean-Michel Cousteau, during a briefing on the whale jail in Srednyaya in May. Photo: Getty

The ABC reported the move to release the whales in their natural habitat was announced last week during President Vladimir Putin’s annual question and answer phone-in.

The Kremlin ordered local authorities in the Russian far east to intervene months ago, but the problem of how to release them without causing them harm caused delays.