Four more vessels have been boarded by thieves in recent days, adding to a spate of raids over the last few weeks.
The IMB's piracy reporting centre listed four more robbers in Asia and South America.
On 3 May, the second engineer on a tanker underway eight miles off Pulau Kukup, Malaysia, noticed five robbers in the engine room.
The alarm was raised and the thieves escaped.
On 2 May, a product tanker at anchorage in Chittagong, Bangladesh, was boarded by two men who were spotted on the poop deck.
