By Gordon Corera
BBC security correspondent
After a period of relative quiet following December's tsunami, maritime piracy appears to be re-emerging in Asia and is sparking concerns about a potential terrorist attack.
The Malacca Strait is a hot spot for piracy
On the evening of 30 April, in the Bangka Strait off Indonesia, six robbers armed with long knives and guns boarded a container ship on the move.
The master of the ship raised the alarm and mustered his crew, fortunately managing to force the boarders to flee.
The incident was only a small example of a worrying trend.
Piracy in the waters of Asia has been a problem for a number of years.
The tsunami led to a drop in the number of incidents in January and through most of February.
This may be partly because the tsunami put some smaller pirates out of business, and also because of the presence of large number of military vessels delivering aid in the area.
And with them the attention of the world's media.
But since the end of February, experts say there has been a notable increase of activity in the Malacca and Singapore Straits.
There have actually been fewer attacks, but they appear to be larger in scale and more organised, with a much greater show of force.
"Attacks are running at one or two a week down from six to 10, but they are all on a larger scale and more professional," said Dominic Armstrong, head of research and intelligence at the security firm Aegis Defence Services.
The new pirates appear more organised and better armed.
They often attack in larger formations, sometimes with flotillas of up to seven boats.
Some vessels have electric fences or high pressure hoses to try and fend off pirates, but without these a simple grappling hook is often enough to get on board the relatively lightly crewed ships.
The crew are not usually armed because vessels have to travel through so many different territorial waters where weapons are not allowed to be carried.
However, a new service is emerging which offers armed escort boats to vessels willing to pay the steep fees.
There is also a trend towards kidnapping captains and other senior officers to demand ransom payments, which in many cases renders the ships involved inoperable.
"There have even been reports of pirates carrying business cards," said Mr Armstrong.
There is concern that because payments have been made in some cases, this could set a dangerous precedent and encourage the practice further.
The trend could become a political and diplomatic issue, especially for Japan whose vessels have suffered heavily and where there is growing pressure to get other countries in the region to increase security and do more to stop piracy.
There are also fears that the Malacca Strait could be the target of terrorists hoping to paralyse global trade - perhaps by seizing an oil tanker and using it as a vast explosive device, in the same way planes were used in the 11 September attacks on the US.
Between a quarter and a third of the world's sea trade goes through the strait. Over the weekend, US Navy Seals were practising anti-terrorism drills with Indonesian forces.
This included practising boarding vessels and fighting pirates as part of an attempt to improve co-operation and prevent an attack.
US Deputy Secretary of State Robert Zoellick offered to help do more to ensure security in the Malacca Strait in a visit to Malaysia on Monday.
In such an important but also internationally sensitive area, greater co-operation may be vital in improving security, but has
so far proved halting due to suspicions, conflicting responsibilities and territorial claims.
But as the sophistication of attacks grows, the pressure will be on for security measures to keep pace.