Armed pirates struck at three ships in the eastbound lane of the Singapore Strait within a span of six hours over Sunday and yesterday.
As the perpetrators have not been caught, they may strike again, said the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP) Information Sharing Centre.
There was no confrontation during the incidents and none of the crew were injured, it added.
Ship equipment was stolen from one vessel, and a lifebuoy from another, while nothing was taken from the third ship.
The latest armed incidents take the total number of such attacks this year in the Singapore Strait to 31, the same number of incidents for the whole of last year.
The first occurred at 9.25pm on Sunday, as tanker Zeno was travelling near Tanjung Bulat in East Johor, Malaysia. Three pirates were spotted in the engine room.
Shipping equipment, including a sounding rod and a portable breathing apparatus, were later found stolen from the engine room.
At 2.20am yesterday, two pirates were spotted near the engine room on bulk carrier Lefkada, which was travelling near Tanjung Ramunia in East Johor, Malaysia.
The alarm was raised and a search of the ship conducted, but the intruders could not be found.
The oil spill equipment locker was found unlocked and a life buoy had been stolen.
And at 3.08am, one pirate was seen near the steering gear compartment on bulk carrier Atlantic Diana, which was near Tanjung Pergam in Pulau Bintan, Indonesia. The crew searched the ship but could not find the intruder. They found nothing stolen.
ReCAAP warned: "All ships are advised to exercise utmost vigilance, adopt extra precautionary measures and report all incidents immediately to the nearest coastal state."
It also advised littoral states to increase their patrols and enforcement, and to increase coordination and information sharing to apprehend the perpetrators.