Attacks on the world's seas are soaring as armed and dangerous pirates become increasingly emboldened, seizing more ships than before and taking even bigger risks, an international body said Thursday.

In the first six months of 2011, there were 266 piracy attacks compared with 196 incidents over the same period last year, and 60 percent of them were carried out by Somali pirates, the International Maritime Bureau (IMB) said.

At the end of June, Somali pirates were still holding 20 vessels and 420 crew, and demanding millions of dollars for their release, the IMB's piracy reporting centre said.

IMB director Pottengal Mukundan warned ship owners and captains to be on their guard as pirates are using new methods and even carrying out attacks in bad weather.

Five years ago, pirates were more often armed with knives. But now ships, including oil and chemical tankers, are being attacked with automatic weapons and rocket propelled grenade launchers, the report said.

Ahead of a UN contact group meeting on piracy at the United Nations in New York later Thursday, the report said many of the attacks were concentrated in the Gulf of Aden, which is criss-crossed by oil tankers.

Fourteen ships had been attacked in the South Red Sea since May 20.

And although Somali pirates were more active with 163 attacks this year up from 100 in the first half of 2010, they had been less successful in actually hijacking ships thanks to the vigilance of international naval forces.

They only managed to capture 21 vessels in the first half of 2011, compared with 27 in the same period last year.

The report from the London-based IMB also said it was vital international forces off the east coast of Africa be kept in place and even boosted.

Worldwide 495 sailors were taken hostage by pirates, who killed seven people and injured 39. Somali pirates took 361 people hostage.

There was also a surge in particularly violent and highly organized attacks off the coast of West Africa this year, says IMB's piracy report.

Twelve tankers have been attacked off Benin since March, an area where no incidents were reported in 2010.

In neighbouring Nigeria, the IMB piracy reporting centre was informed of three boardings, two vessels being fired upon, and one attempted attack.

But IMB says the seas around Nigeria are more dangerous than the official reports suggest, and that it was aware of at least 11 other incidents that had not been reported.

Overall, 50 incidents were recorded for Indonesia, Malaysia, Singapore Straits and the South China Seas in the first two quarters of 2011.