Militants from ISIS in Libya abducted nine foreigners, including four Filipinos, in an attack on a central oilfield last week and beheaded eight Libyan guards, officials said on Monday.

The attack last Friday on the al-Ghani oilfield near the town of Zalla, 750km (470 miles) south-east of the capital, Tripoli, was part of a series of deadly assaults on Libya’s oil infrastructure by ISIS.

The attacks in recent weeks have forced Libya to declare 11 fields non-operational, including al-Ghani, and invoke a force majeure clause that exempts the state from contractual obligations.

Libya’s military spokesman, Ahmed al-Mesmari, warned on Monday that ISIS militants’ long-term goal is to take over Libya’s petroleum industry. “This is the lifeline of the Libyan people,” he said.

During the attack on al-Ghani, an employee watched the beheadings of the eight oil guards and subsequently died of a heart attack, al-Mesmari also said. He did not elaborate on how the army knew 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.

Meanwhile, authorities in the Philippines said four of their nationals were among the nine abducted from the oil field. An Austrian, a Czech, a Bangladeshi and a Ghana national were also taken. One hostage remains unidentified.