Somali pirate attacks are increasing, spurred by a 36-fold jump in ransoms in five years, raising costs for shippers and the threat to vessels carrying 20 percent of world trade.

The raids are adding at least $2.4 billion to transport costs because vessels are being diverted onto longer routes to avoid attacks off east Africa, Louisville, Colorado-based One Earth Future Foundation estimates. Average ransom payments rose to $5.4 million last year, compared with $150,000 in 2005, the non-profit group says. Attacks off Somalia were the highest on record last year, with 49 vessels and 1,016 crew members hijacked, according to the International Maritime Bureau.

"It's likely to continue on its trajectory," said Roger Middleton, an analyst covering Somalia at the Royal Institute of International Affairs in London. "Every time some of them are arrested, there are plenty of others happy to take their places because it's so well paid."

Escalating attacks are driving some tankers to sail around southern Africa rather than through the Suez Canal, adding about 12 days to a journey from Saudi Arabia to Houston, said Luis Mateus, an analyst at Riverlake Shipping SA, a broker in Geneva. Returns for owners on the route averaged $11,743 a day last year, according to data from the Baltic Exchange in London, which publishes assessments for more than 50 maritime routes.

About 30 anti-piracy ships are deployed daily in the region by groups including the European Union and NATO. The European Naval Force patrols about 2 million square nautical miles, or an area 10 times the size of Germany.