The marine mammals were in a good shape, an official statement said

A number of whales and orcas captured to perform in aquariums and held in cramped pens have been released into the wild, but experts warned the mammals may not survive after being held in captivity for months.

The release of six beluga and two killer whales into the Sea of Okhotsk in Russia's far east came after a huge international outcry over the holding in captivity of nearly 100 of the marine mammals.

However, scientists and activists criticised the move, saying it was not transparent and did not follow a previously agreed roadmap. They also warned the orcas may not survive in the wild.

It is also unclear what authorities plan to do with the remaining animals, still held near the far eastern town of Nakhodka.

"The release took place in favourable weather conditions," said the All-Russian Fisheries and Oceanography Institute (VNIRO), which led the operation.

"The release took place in favourable weather conditions," said the All-Russian Fisheries and Oceanography Institute

The marine mammals were in a good shape, it said in a statement.

"The orcas were predictably jittery, they spent several hours near the shore and then entered the Gulf of Sakhalin's open waters," the institute said.

They have been tagged to monitor their whereabouts, it said.

The juvenile animals have been held in cramped enclosures since last summer by commercial companies that planned to deliver them to aquariums, including in China where the industry is booming.

Animal welfare groups, scientists and celebrities had expressed horror at their fate and repeatedly called for their release.

Russia is the only country capturing wild orcas and belugas and selling them to aquariums, a controversial practice that has continued due to legal loopholes.

The controversy came to a head when images of what the media have nicknamed the "whale jail" were published this year.

Marine mammal experts have advised a process of pre-release rehabilitation for the animals following the prolonged period of human contact, a difficult winter and health problems of a number of the orcas.

'Few chances of survival'

During his annual phone-in with ordinary Russians last week, President Vladimir Putin said he was in favour of the whales' release.

"Thank God things have started moving," he said.

But Russian killer whale experts, apparently uninvolved in the release, said the chances of survival were fairly slim.

"The experts have recommended that all orcas are released together after rehabilitation. However, VNIRO released only two of them without any rehabilitation," said a statement posted on Russian Orcas Facebook group run by researchers.

"We hope for the best for them, but the way they were released leaves them with few chances of survival," it said.

Orca expert Tatiana Ivkovich said there were no known marine mammal scientists involved in the release which saw the animals transported in trucks for nearly a week.

"How will we know what happened to the two orcas?" she asked. "Who is in those trucks?"

Greenpeace Russia criticised the "secretive atmosphere" of the operation and complained that no media were allowed to observe or count the animals during transport.

The four companies keeping the animals were given hefty fines by a Russian court this month for violating fishing regulations.

Environmentalists have raised concerns that the same companies have apparently been paid to organise the release, after Russian media published details of the contracts.

The Russian navy previously said it would help transport the animals for free.

French marine expert Jean-Michel Cousteau, who visited the country earlier this year to agree a plan for the whales with officials and scientists, on Wednesday called on Russia to work with global specialists on the release, which is unprecedented in its scope.

"This operation is so complicated and so important that Russia must have the best specialists," he said. "I am hearing concerns from the public about the lack of transparency that we have seen so far."

VNIRO said it had taken six days to transport the animals to the release site some 1,800 kilometres away from the enclosure.

The institute said the mammals were continuously monitored and that "not a single animal was harmed during the journey."