The number of attacks rose to 240 between January and June, up from 114 incidents in the same period a year ago, according to a report released by the International Maritime Bureau's piracy reporting center in Kuala Lumpur.
Ships were boarded in 78 cases and 31 vessels were hijacked, with 561 crew taken hostage, 19 injured and six killed, the bureau said in its quarterly report. The attackers were heavily armed with guns or knives in most of the cases, it said.
The higher attacks were due mainly to increased Somali pirate activity off the Gulf of Aden and east coast of Somalia, which combined accounts for 130 of the cases, the report said.
Somalia has not had an effective government since 1991 when the overthrow of a dictatorship plunged the country into chaos. Besides frequent land battles, the power vacuum has also allowed pirates to operate freely around Somalia's 1,900-mile coastline.
The International Maritime Bureau said Somali attacks peaked in March and April, with no attacks recorded in June. The recent decline was largely because of monsoon-related poor weather that is expected to continue through August, the report said.
"Vigilance should nevertheless remain high during this period," the center said.
International navy patrols in the gulf have also helped to thwart pirate activity, though military vessels are hard-pressed to cover the vast expanse of ocean along Somalia's coastline.
Among other nations that reported significant attacks in the six months, Nigeria had 13, Peru reported 10, Malaysia had nine and India had six.