A ceremony has been held in the Malacca Straits to mark the beginning of co-ordinated naval patrols between Indonesia, Singapore and Malaysia.
The armed forces commanders of all three nations stood in salute as 17 ships began patrolling the waterway.
The three countries all border the narrow Malacca Straits, one of the busiest shipping lanes in the world.
The patrols are a response to fears that the straits are vulnerable to attack from pirates or terrorists.
To mark the new agreement, ships fired flares into the sky, and parachutists dropped from a plane into the water to perform a simulated pirate chase.
All three countries will contribute up to seven ships to the patrol, with each ship remaining under its own nation's commands.
Vital trade link
From the point where the Malacca Straits are at their narrowest, the Indonesian island of Batam can clearly be seen on one side and the skyscrapers of Singapore on the other.
In between there are a huge number of ships, tankers, freighters and tugs.
The Malacca Straits are notorious for pirate attacks
The narrow waterway carries more than a quarter of the world's trade, and almost all the oil imports destined for Japan and China.
Piracy has been a problem for centuries, but it has been getting worse in recent years. Since the 11 September attacks on the US,
the threat of attacks in the region has also increased.
Our correspondent in Indonesia, Rachel Harvey, says the policing of the Malacca Straits is a sensitive diplomatic issue - in an increasingly sensitive security situation.
Indonesia's navy chief, Admiral Bernard Kent Sondakh, recently said in an interview with Tempo magazine that foreign governments - including the US - were primarily interested in the waterway because it was economically strategic, rather than because of terrorism fears.
In the same interview, he accused the International Maritime Bureau - an independent body to protect international shipping - of over-reporting the amount of piracy in the region.
But Singapore is still holding open the possibility that it may need more help in patrolling its coasts in future, possibly from the US.