Pirates have released two Indonesian sailors taken hostage last weekend in the Malacca Strait.
An Indonesian navy spokesman said the men were the captain and engineer of the gas tanker Tri Samudra.
The spokesman said the hostage-takers had not been identified, and he did not know whether a ransom was paid.
Search-and-rescue teams are still looking for the captain and two crewmen of a Japanese tugboat, kidnapped in the Malacca Strait on Monday.
Around a quarter of the world's trade and half of its oil supplies pass through the Malacca Strait, which is considered a hotspot for piracy.
Malaysian arrests
The two Indonesians were kidnapped on Saturday or Sunday last week when a gang of more than 30 pirates armed with machine guns and rocket launchers boarded the Indonesian-registered ship.
The kidnappers demanded a ransom of two billion rupiah ($211,449, £110,000) from the owners of the ship.
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 the Japanese captain and engineer, and a Filipino crewman were captured.
On Friday Malaysian marine police said they had detained four Thais and a Malaysian suspected of involvement in the assault.
The Indonesian navy spokesman said the whereabouts of the hostages are still not known, but added: "We hope that with the arrest in Malaysia they will soon be found."
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