A maritime watchdog on Friday issued a warning to international seafarers of an increasing number of pirate attacks this week in the Gulf of Aden and off the east coast of Somalia, dpa reported.
The London-based International Maritime Bureau, warned all ships passing through the troubled waters to "maintain a strict 24-hour visual and radar watch" against pirates.
"In the past two days, pirates have been actively attacking vessels with intent to hijack," said the statement, released by the bureau's Kuala Lumpur-based reporting centre.
The bureau blamed the increase in pirate activities on favourable weather.
Five armed attacks by pirates took place in Somali waters Thursday and Friday, the bureau said.
The pirates aborted all five attacks after crew members took evasive measures or sent out calls for assistance.
The Gulf of Aden, one of the world's busiest waterways, remains a hotspot for pirate activity despite increased calls for joint security patrols in the region.
Around 40 ships were hijacked last year, prompting NATO, the European Union, India, Russia and other countries to deploy warships in the region.
Pirates were still holding 10 ships and more than 200 crew members.