Heavily armed Somali pirates attacked six ships, including British, Indian and Singapore-managed vessels, earlier this week but all managed to escape, a global maritime watchdog said on Friday.
"In the past two days, pirates have been actively attacking vessels with intent to hijack," Noel Choong, head of the International Maritime Bureau piracy reporting centre in Kuala Lumpur, said in a statement.
"It appears that favourable weather conditions in the area and the high number of hijacked vessel that have been released recently may have prompted the pirates to actively seek for new targets," he added.
But the ships -- managed by Indian, British, Greek, Singapore and Philippine companies and one unknown -- managed to escape from the pirates.
Choong said the strong presence of naval warships in strategic locations had prevented successful hijackings in recent weeks.
"The number of successful hijackings has been reduced due to naval activities," he told AFP.
Choong urged ships to maintain 24-hour visual and radar watches to prevent hijacks.
In one attack on Thursday in the Gulf of Aden, Choong said Somali pirates in a speed boat opened fire on a Indian-managed ship.
"The master contacted naval warships. The pirate's boat came close to the vessel but aborted the attempt due to evasive actions taken by the vessel," he said.
In another dramatic attack, pirates fired rocket-propelled grenades (RPGs) at a Singapore ship in the Gulf of Aden.
"The ship's master reported that pirates fired rocket-propelled grenades at the vessel. A military helicopter responded to its distress call," he said.
Choong said six pirates attacked a Philippine ship on Wednesday off Somalia.
"Six pirates armed with RPGs and guns in a speed boat chased and fired their RPG at the bulk carrier. The vessel took evasive manoeuvres and escaped from the pirates," he said.
Choong said since January 2009, there had been 22 attacks, with seven vessels and 123 seamen being held by Somali pirates.
Japanese warships are expected to soon join a growing fleet of foreign navies patrolling what have become the world's most dangerous waters, with more than 130 pirate attacks in 2008 alone.
According to experts, a slightly lower rate of successful pirate attacks since the start of 2009 is due to a combination of weather conditions and increased navy surveillance.