Pirates operating off the coast of east Africa have hijacked a Saudi supertanker fully laden with an estimated 2m barrels of oil in an attack that marks a significant escalation in the scope of banditry in the region.

The pirates, believed to be from lawless Somalia, seized control of the Sirius Star, which is owned by Saudi Aramco, the world's largest oil company, on Saturday, 450 nautical miles south-east of the Kenyan Indian Ocean port of Mombasa.