Pirates on a speedboat followed the large Japanese ship and attempted to board it from the back, Noel Choong, from the London-based International Maritime Bureau, said on Tuesday.


However, after a ship duty officer raised the alarm the crew turned on floodlights and sprayed water from fire hoses, preventing the pirates from boarding.


The pirates, thought to be armed, chased the ship for five minutes before giving up, Choong said.


On Sunday night, pirates also boarded two UN-chartered ships carrying building material for the reconstruction of Indonesia's tsunami-hit Aceh province.


No injuries were reported among the all-Indonesian crew aboard the two vessels, but cash and some materials were stolen.


"At the moment we don't know if these are isolated cases or the start of attacks again in the Malacca Strait," Choong said.


"We are still monitoring [and] we urge ships to keep a strict piracy watch."


The Malacca Strait is one of the busiest sea lanes in the world, and has many pirate attacks.


In recent years the three states bordering the waterway - Malaysia, Indonesia and Singapore - have started joint measures to combat piracy, including air and naval patrols.