Libya's military spokesman says militants from the country's Islamic State affiliate beheaded eight guards after an assault on an oil field last week during which nine foreigners were abducted.

The spokesman, Ahmed al-Mesmari, also said Monday that an employee of the al-Ghani oil field watched the beheadings and subsequently died of a heart attack.

Al-Mesmari did not elaborate on how the army knows about the beheadings but the force serving as oil guards is closely allied to the Libyan military, which answers to the eastern-based government, one of Libya's two rival governments.

Authorities in the Philippines and Austria confirmed that nine of their citizens were abducted during the attack Friday on al-Ghani, part of IS attacks on Libya's oil infrastructure.