Nato is considering a significant extension of its anti-piracy mission off Somalia amid growing concern within the 26-member alliance at the risk pirates pose to international trade and security.

For the past six weeks Nato forces have deployed four ships in anti-piracy patrols in the Gulf of Aden, the perilous stretch of water separating Yemen and Somalia. Those are due to withdraw this month when seven naval vessels under European Union command are deployed in the region.