A group of Somali pirates captured by the Russian navy in the Gulf of Aden and then released are likely dead after failing to reach the shore, a top Russian defence source said Tuesday.

The 10 pirates were captured last week after seizing a Russian oil tanker but were then unexpectedly released, with Russian officials saying there was insufficient legal basis to keep them in detention.

"According to the latest information, the pirates who seized the 'Moscow University' oil tanker failed to reach the shore. Evidently, they have all died," the source was quoted as saying by Russian news agencies.

The Interfax news agency said after their release the Russian navy put the pirates in a boat some 300 nautical miles offshore, removing all weaponry and navigational equipment from the vessel.

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