A security team boarded the Marshall Islands-flagged Royal Diamond 7 in international waters about 150 kilometres (93 miles) north of the Libyan city of Derna, the European Union's military operation in the Mediterranean said.

The ship, which had earlier left Sharjah in the United Arab Emirates, was heading for Benghazi "carrying a load of jet fuel likely to be used for military purposes," Operation Irini said in a statement.

"This fuel is considered 'military material' by the UN," it added, saying the Royal Diamond 7 was denied access to Libyan territorial waters.

It has been "diverted to a European Union port for further investigation," the mission said.

Operation Irini was set up to halt the flow of arms into Libya, where the UN-recognised and Turkish-backed Government of National Accord has faced a major uprising by the forces of Russian-backed strongman Khalifa Haftar.

Since starting on May 4, patrols in the central Mediterranean have hailed 650 vessels and made 12 searches on board merchant boats, the mission said.

It has also detected some 80 suspected flights "possibly carrying military-related cargos back-and-forth to Libya," it said.