Armed pirates stormed three ships in the Singapore Strait in the span of 2 ½ hours between Sunday night and yesterday morning.
But nothing was stolen from the bulk carriers in the three separate incidents, which took place in close proximity to one another in the eastbound lane of the Traffic Separation Scheme (TSS) in the waterway.
Neither were their crew injured, the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP) Information Sharing Centre said yesterday.
The latest armed incidents bring the tally for this month to six, the highest monthly figure this year.
Altogether, 28 such incidents in the Singapore Strait have been reported so far this year, against 31 for the whole of last year.
Of this year's cases, 24 occurred in the eastbound lane of the TSS.
The 105km-long Singapore Strait is significant as it provides passage for thousands of ships entering and leaving the port of Singapore.
In all of this month's attacks, there was no confrontation between the perpetrators and the crew.
Nothing was stolen except in one incident, where a lifebuoy and some welding rods were taken.
In the latest three cases, the first took place at about 11.09pm on Sunday, when bulk carrier Seajourney was travelling near Nongsa Point in Batam, Indonesia, on the way to Ciwadan, also in Indonesia.
The ship master reported to the Vessel Traffic Information System (VTIS) that an unauthorised perpetrator had been spotted near the engine room entrance.
The ReCAAP Information Sharing Centre (ISC) said: "The alarm was raised and the master carried out a search on board the ship, but with no sighting of the perpetrator. The crew was safe, and nothing was stolen."
The Republic of Singapore Navy's Maritime Security Task Force (MSTF) and the Singapore Police Coast Guard were notified, along with the Indonesian authorities.
A safety navigational broadcast was also made.
About two hours later, at 12.46am yesterday, another bulk carrier, A Racer, was travelling north of Pulau Nongsa, Indonesia, on its way to China, when it was alerted by VTIS to an unknown craft alongside it.
The ship master reported that five unauthorised persons had been seen on board. The alarm was raised and a search carried out.
There was no sighting of the perpetrators, nothing was lost and the crew were unharmed.
The Singapore navy's MSTF, the Police Coast Guard and the Indonesian authorities were notified.
Less than an hour later, at 1.30am, another bulk carrier, El Matador, was near Nongsa Point in Batam, when it was alerted to an unknown craft alongside it.
The ship master spotted three intruders in the engine room, raised the alarm and four perpetrators were seen fleeing the vessel.
As he was unsure if all the intruders had left the ship, the ship master diverted the vessel to the anchorage in Batam, where the Indonesian navy boarded and searched the ship.
There were no more perpetrators on board, the crew was safe and nothing stolen, so the ship resumed its voyage.
"The ReCAAP ISC is concerned with the increase of incidents in the Singapore Strait. As the perpetrators of these incidents have not been arrested, there is a possibility of further incidents in the Singapore Strait," the organisation said.
"All ships are advised to exercise utmost vigilance, adopt extra precautionary measures and report all incidents immediately to the nearest coastal state."
The ReCAAP ISC also urged the littoral states to increase patrols and enforcement in their respective waters, and further coordinate and share information about incidents and criminal groups involved.