LONDON: Somali pirates seizing Indian Ocean ships were responsible for at least 35 hostage deaths in 2011, a report showed on Friday, with levels of violence rising.

The number of prisoners taken by pirates fell to 555, at least, in 2011 from 645 in 2010, the report by the US-based One Earth Future foundation and International Maritime Bureau said.

Eight were known to have been killed by their captors either during their initial capture or were executed later, it said, with another eight dying of malnutrition or disease. The remainder were killed either during rescue attempts by military forces or while trying to escape.

While solid data on previous years is limited, the total of 35 is seen as by far the highest number of piracy-related fatalities in a single year.

“We know these figures are almost certainly an underestimate,” project manager Kaija Hurlburt told Reuters. “A lot of the ships now being taken are regional dhows that are often never reported. They might have 12 to 20 people aboard each time.”

Despite a major naval effort by several nations, hundreds of young Somalis engage in piracy every year in the hope of ransoms that can run to millions of dollars.

With some shipowners apparently simply abandoning their vessels and crews, particularly the smaller more vulnerable craft, crews have found themselves held for ever longer periods.

As more and more merchant ships carry armed guards, foreign navies take tougher action and some shipowners prove unable or unwilling to pay up, some believe piracy itself is getting harder - and that is being taken out on those in captivity.

At least 149 hostages had now been held for more than a year, the report said, with 26 held for more than two years.

Many of those released reported abuse including beatings, removal of fingernails and dumping in the sea.

More than 40 per cent said that at some stage they had been used as human shields, often when pirates sailed captured vessels back out to sea to act as mother ships for new attacks.

Most hostages were from developing countries, particularly the Philippines, India and China as well as Gulf and African states.

The level of violence being used was also increasing, the report said. In 2011, more than 3,800 personnel were aboard ships that were attacked by pirates with firearms in what were often prolonged and brutal assaults.

Casualties among the pirates were also almost certainly on the rise, with reports of at least 111 killed in 2011, some 70 per cent in clashes with increasingly aggressive naval forces.