In the past fortnight eight vessels including the Saudi supertanker Sirius Star loaded with oil worth £60m have been seized in increasingly daring attacks. Several hundred crew are now in the hands of the pirates.

Kenyan Foreign Minister Moses Wetangula said yesterday: "We are advised that in the last 12 months ransom to the excess of $150m has been paid to these criminals and that is why they are becoming more and more audacious in their activities."

Saudi Arabia's Foreign Minister warned the only way to deal with pirates like those holding the Sirius Star is "by eradicating" them.

Prince Saud Al-Faisal said piracy was "not something you can negotiate or justify in any way, means, or manner". "Like terrorism, it is an evil that has to be eradicated. There is an international consensus towards endeavours to do just that," he said.

Meanwhile, the world's largest oil tanker company warned that it may divert cargo shipments, which would increase costs by up to 40 per cent. Frontline, which ferries five to 10 tankers of crude a month through the treacherous Gulf of Aden, said it was negotiating a change of shipping routes with some of its customers, including oil giants Exxon Mobil, Shell, BP and Chevron.

Chief executive Martin Jensen said sending tankers around South Africa instead would extend the trip by 40 per cent.

Bermuda-based Frontline plans to make a decision whether to change shipping routes within a week. Mr Jensen said: "It's not only our costs, but also those of the people who have a $100m cargo on board. We're not going to make a unilateral decision so we've been debating this with our customers."

On Thursday AP Moller-Maersk, the world's largest container-shipping company, ordered some of its slower vessels to avoid the Gulf of Aden and head the long way around Africa.

The Copenhagen-based company said it was telling slower ships to take the long route unless they can join convoys with naval escorts.