Sea piracy fell 54 percent worldwide in the first half of 2012, led by a dramatic drop in Somali piracy, an international maritime watchdog said Monday.

The International Maritime Bureau attributed the sharp drop to anti-piracy operations by international navies patrolling in seas off Somalia as well as increased vigilance by ships, including the hiring of private armed guards on board.

The bureau said 177 attacks were reported worldwide from January to June, down from 266 in the same period last year. It said 20 vessels were hijacked, with 334 crew members taken hostage and at least four crew members killed.

Attacks off Somalia's coast plunged to 69 in the first six months from 163 a year earlier, it said. Somali pirates were able to seize 13 vessels, down from 21.

"The naval actions play an essential role in frustrating the pirates. There is no alternative to their continued presence," said IMB Director Pottengal Mukundan. He warned that Somali pirates remain a serious threat, with 11 vessels and 218 crew members still in their hands as of late June.