ALBANIAN officials say they are investigating how 800 tonnes of alleged hazardous industrial waste was shipped from its port without authorization, bound for Southeast Asia, as the suspect cargo is due to be sent back.

Meanwhile, an AP Moller-Maersk A/S ship transporting some of the suspect cargo offloaded the containers in Singapore over the weekend, having initially sailed past the port before doubling back to dock in an unusual move.

Some 40 containers that environmental groups allege carry toxic dust waste were set to be transferred last Monday onto an MSC Mediterranean Shipping Company SA vessel that will ferry them back to Europe, according to MSC's marine cargo tracking website.

Maersk said it's working with Singaporean authorities and MSC to return the cargo to Albania, reports Ventura, California's gCaptain.

For now, the containers are set to arrive at Italy's Gioia Tauro port in late September, according to the tracking data.

Another 60 containers of suspected waste which is currently on board another Maersk container ship - due in Singapore later this month - will also head back to Europe, Maersk said.

Singaporean officials said they are "facilitating the return of these containers" under the United Nations Basel Convention, a global pact signed by many developed economies that requires countries to give consent for waste headed their way.

Officials have been working to stop the shipment since Basel Action Network (BAN), a US-based nonprofit that tracks toxic trade, last week informed Thailand that the containers it believes are filled with potentially harmful electric arc furnace dust were heading its way.

BAN has called for the shipments to be sent back to Albania and the traders involved punished.