By Jonathan Kent
BBC, Kuala Lumpur
Malaysia has promised to deploy more ships to the Malacca Strait after pirates kidnapped several crewmen for ransom in two attacks this week.
A Japanese tugboat was attacked on Monday
However, the authorities say they will not reconsider offers of help from other countries, notably the United States, to patrol the strait.
Malaysia has launched an air and sea search and rescue operation to locate the missing mariners.
A lull in piracy in the strait after December's tsunami appears to be over.
On Monday, at least 10 pirates opened fire on a Japanese-flagged tug, the Idaten, and boarded it.
Eleven crew members made it safely to the Malaysian port of Penang, but two Japanese and a Filipino were seized by the attackers, probably for ransom.
Two days earlier, an Indonesian tanker, the Tri Samudra, was hijacked by more than 30 heavily armed men. The captain and first engineer are still missing.
The suspicion is that pirates from the tsunami-hit region of Aceh are operating once again.
The authorities say they are sure that the kidnap victims were taken to the Indonesian island of Sumatra.
The International Maritime Bureau's piracy reporting centre in Kuala Lumpur attributes the earlier respite from attacks not just to the tsunami, but also to the presence of dozens of foreign naval vessels that came to help.
Around a quarter of the world's trade and half of its oil supplies pass through the strait, which is considered a hotspot for piracy.
Much of the blame has been laid at Indonesia's door for failing to patrol its side of the channel properly.