Ten Somali pirates captured then freed by Russian forces have mysteriously died, it emerged yesterday.

An unidentified high-ranking Russian Defence Ministry official did not explain how the pirates died, prompting speculation they were executed by commandos who had stormed a captured oil tanker 500 miles off Somalia's coast.

The official told Russian news agencies the pirates' boat disappeared from Russian radar about an hour after their release.

"They could not reach the coast and, apparently, have all died," the official said.

Russian officials have said one of the 11 pirates was killed during a gun battle when the special forces took the tanker last Thursday. The others, some said to have been wounded, were put on board a Russian destroyer.

Officials initially said they would be taken to Russia for trial, but the Defence Ministry said on Friday they had been released because of "imperfections" in international law. The statement was met with scepticism, especially in light of a comment made by the Russian President.

"We'll have to do what our forefathers did when they met the pirates" until the international community comes up with a legal way of prosecuting them, Dmitry Medvedev said on the day the ship was stormed.

The international community has had difficulty formulating an accepted policy for trying suspected pirates. Somalia's ambassador to Russia, Mohammed Handule, told journalists that his government could not identify or locate the pirates. They were believed to be Somalis, but their nationality has never been confirmed.

He said they "will face trial if we find them alive."

The multi-million dollar business of pirate attacks has continued to climb despite the presence of about 35 international warships patrolling the waters off Somalia. The impoverished nation is caught up in an Islamic insurgency and has not had a functioning government since 1991.