It attributed the rise to the expansion of the global sea trade, congested chokepoints, corrupt officials, shifting spending priorities, lax coastal and port security, and the availability of small arms.

"Combined with the large number of ports around the world, this growth has provided pirates with an almost limitless range of tempting, high pay-off targets," said the study by the RAND Corporation, a nonprofit organisation.

The study, which was authored by Peter Chalk, said 2,463 actual or attempted acts of piracy occurred between 2000 and the end of 2006, nearly a quarter of them in waters around the Indonesian archipelago.

The average annual rate of incidents during those years was 68 per cent higher than in the previous six-year period, it said.

But it said fears that extremist groups would use pirate gangs and networks to move material or fighters or to carry out attacks have not materialised.

"To date, there has been no credible evidence to support speculation about such a nexus emerging," the report said.

"Just as importantly, the objectives of the two actors remain entirely distinct."

Nevertheless, the report said, governments, international organisations and major shipping interests remain concerned that extremist groups could exploit the same vulnerabilities in commercial maritime trade that attract pirates.