(AFP) An Indonesian tanker laden with a toxic chemical has capsized off Malaysia's southern Johor state near Singapore, leaking pollution and killing thousands of fish, officials said today.

The 533-tonne MV Endah Lestari sprung a leak shortly after leaving the Johor port of Pasir Gudang early Wednesday, said the marine director for southern Malaysia, Hazman Hussein.

It was on its way to East Kalimantan in Indonesia with some 600 tonnes of the poisonous industrial chemical phenol, and 18 tonnes of diesel.

Hazman said the tanker's captain alerted the Johor Port Authority on noticing the leak in the narrow Tebrau Straits separating Malaysia and Singapore.

Two tugboats were despatched and towed the vessel to near Pasir Gudang where it capsized, he said. All 13 crew members were safe.

Hazman said there was some spillage of diesel and of the chemical, but the volume was unknown. Newspaper reports said the toxic spill had killed thousands of fish and cockles reared in 85 offshore cages.

"The diesel spillage has been contained and most of the cargo tanks appear secured. We are monitoring the situation very closely," he said, adding that it would be tough to mop up phenol as it is soluble in water.

Hazman said authorities were investigating the cause of the incident and a representative of the tanker owner, PT Lima Bahari Pandunusa, was expected to fly to Johor later today.