ATHENS (REUTERS) - Greek authorities are investigating a possible Islamic State in Iraq and Syria (ISIS) link behind the discovery of a huge quantity of synthetic opiates destined for Libya.

The Narcotics and Arms Division of Greece's Financial Crimes Unit (SDOE), in cooperation with the US Drug Enforcement Administration (DEA), seized 26 million tablets containing Tramadol, a painkiller available only on prescription.

They had arrived at Piraeus, Greece's largest commercial port, in a container from New Delhi on May 10. Greek agents acted on a tip-off from the DEA, said Loukas Danabasis, director of the division.

The pills were found stacked behind boxes of household linen, which was the listed consignment on shipping documents.

Authorities were investigating whether the pills were destined for Islamic militants, Danabasis said.

"The companies that were involved in this case - in other words the company that sent it, as well as the company that was going to receive it in Libya - have been previously involved in such transfers - and particularly the company in Libya is characterised as being suspect of having relations with the Islamic State," he said.

Greek authorities did not name the companies involved in the transfer. The drugs were estimated to have a value of about 13 million dollars.