Pirates have continued to launch attacks in the Singapore Strait, with three reported incidents last month in the 105km-long stretch of water south of Singapore which serves as a key waterway for trade.

Last month's incidents took place north-west of Bintan, Indonesia, in the eastbound lane of the strait. The same lane was plagued last year by 30 reported incidents of piracy.

In a statement yesterday, the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP) Information Sharing Centre said it was "concerned with the continued occurrence of incidents in the Singapore Strait". The Singapore Strait passes through the territorial jurisdictions of the Republic, Malaysia and Indonesia.

It also noted that there was a total of 34 piracy incidents last year, the highest mark in five years.

In the three cases last month, the perpetrators were spotted in the engine room of the vessels, but nothing was reported stolen nor were the crew harmed.

"The three incidents occurred in close proximity to each other and in particular, two incidents occurred within an interval of four hours," said the ReCAAP centre.

At about 3.25am on Jan 25, bulk carrier Ark Royal was passing through the area north-west of Tanjung Pergam, Pulau Bintan in Indonesia, when five intruders were spotted in the engine room. The alarm was raised and a search conducted. At 5.06am, the ship master updated Singapore Vessel Traffic Information System that the intruders had left. With no aid required, the ship continued on its journey.

On Jan 28, bulk carrier Elvia was passing through the same area on its way to China when 10 perpetrators with knives were spotted in the engine room at about 10.32pm.

A search was conducted. After about an hour, the ship master told the authorities that there were no further signs of the intruders, and the vessel continued on its journey.

About four hours later, at 2.21am on Jan 29, cargo ship Vantage Wave was passing through the same location on its way to Caofeidian, China, when three intruders with knives were spotted in the engine room. There were no further signs of them after an hour-long search, and the ship went on its way.

In the cases, the Singapore navy's Maritime Security Task Force and Singapore Police Coast Guard were informed, along with the Malaysian and Indonesian authorities.

"As the perpetrators of these incidents are not arrested, there is a possibility of further incidents occurring in the Singapore Strait," said the ReCAAP centre.

"All ships are advised to exercise enhanced vigilance, adopt extra precautionary measures and report all incidents immediately to the nearest coastal State."

It urged sailors to stay informed about the situation and to heed advisories and navigational broadcasts by the three littoral states. It also called for the three nations to increase patrols and enforcement in their respective waters and further coordinate and share information, in order to arrest the perpetrators.