They come armed with guns, sometimes machetes and even swords, holding ship crews hostage while stealing valuable fuel.
Reported incidents of pirate attacks in the waters surrounding Singapore have been on the rise since the middle of last year, according to the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP).
ReCAAP, which has 20 government members and is headquartered in Singapore, compiles regional incident reports.
In its latest report on fuel and oil siphoning in Asian waters, the organisation noted that 15 incidents were reported aboard oil or product tankers in Asia last year, up from just three in 2013.
Of the 15 incidents, 12 were successful thefts, and eight of the vessels had Singapore as their last port of call.
"There is no clear evidence that Singapore-registered vessels are targeted, but more vessels departing from Singapore seem to be boarded," said ReCAAP deputy director Nicholas Teo.
He said that, although it is not conclusive, one possibility is that the fuel is of better quality and can thus command a better resale price.
Other factors could include the high traffic going through the city-state's port, and more diligent reporting of hijackings, maritime security analysts told The Straits Times.
Just last week, a Singapore-registered tanker under the company Ocean Tankers became the fifth known casualty since January, after eight gun-bearing pirates stormed on board in the Strait of Malacca, off Port Dickson.
The crew members were locked up while the pirates siphoned 2,023 tonnes of gas oil into a barge they had navigated alongside the tanker. The perpetrators damaged the ship's communications equipment so the victims could not send out messages for help. They left the vessel only the following day, after grabbing the crew's cash and mobile phones.
The attackers are believed to have been from Indonesia, said a spokesman for Ocean Tankers. The crew members were not physically hurt in the incident, but underwent counselling.
They are considered lucky, as it is not uncommon for such armed theft to result in casualties, said Dr Phillip Belcher, the marine director of Intertanko.
"What we have here in South-east Asia is pure property theft, in which seafarers are unwittingly being put in harm's way, with several killed," he said.
"It's a commercial operation... but seafarers are getting in the way, suffering fatal injuries."
And with a lucrative market for fuel, the attacks are expected to continue.
There is "corruption and distortion in the fuel distribution system, particularly in Indonesia", said Dr Sam Bateman, an adviser to the Maritime Security Programme at the S. Rajaratnam School of International Studies. "The relative difference in fuel prices between regional countries is another factor leading to a regional 'black market' in fuels."
Mr Karsten von Hoesslin, the special projects manager of Risk Intelligence, called fuel "the perfect product to steal" because "it is nearly untraceable".
The pirate syndicates are typically involved in other illegal activities at sea, with primarily Indonesian "foot soldiers".
The "middlemen and big bosses", however, are "generally Malaysian and Singaporean", noted Mr von Hoesslin.
The stolen product is either sold to other vessels or blended and sold onwards internationally, making it difficult to track.
"The money often ends up in a 'Big Boss' Singaporean bank account," said Mr von Hoesslin.
"Singapore is a lucrative market. It is the No.1 bunker market in the world and, unfortunately, also known for a darker side to its bunkering legacy... Syndicates have acknowledged that bunkering agents in Singapore are often the 'insider information' link that is compromised when it comes to the security of the vessel and cargo."
More efforts to curb fuel- siphoning attacks by pirates must be made by both tanker companies and the regional authorities, said Dr Bateman.
"Largely because there are so many of these small product tankers around the region, I suspect that inspection and security regimes for these ships are not quite as strict as those for larger vessels," he said.
"Closer co-operation is required between all regional maritime and law-enforcement agencies, including shore police forces and Customs agencies - recognising that the siphoned-off fuel has to be brought ashore somewhere."
This article was first published on May 11, 2015. Get a copy of The Straits Times or go to straitstimes.com for more stories.