The number of attacks by pirates worldwide has fallen in the last year but armed robbery and kidnappings at sea have surged off the coast of west Africa, a maritime body said on Monday.
Pottengal Mukundan, director of the International Maritime Bureau (IMB), urged west and central African leaders to act on an agreement reached last month to tackle the problem.
"This (code of conduct) should be translated soon into action on the water. If these attacks are left unchecked, they will become more frequent, bolder and more violent," he said.
"Cooperation and capacity building among the coastal states in this region is the way forward and urgently needed to make these waters safe for seafarers and vessels."
In the first six months of this year, the London-based IMB's Piracy Reporting Centre recorded 138 incidents worldwide, compared to 177 in the same period in 2012.
Hijackings fell from 20 to seven so far in 2013, while the number of sailors taken hostage fell from 334 to 127, the quarterly report said.
Attacks off the coast of Somalia have dropped "significantly" in the first half of 2013, largely due to increased military action, the IMB's report said.
But it warned of increased pirate activity in the Gulf of Guinea, recording 31 incidents in the region -- 22 of which took place off the coast of Nigeria.
There has been a surge of kidnappings at sea and pirates are targeting a wider range of ship types in a region already known for attacks on oil industry vessels, the report said.