Nine foreign oil workers have been seized by Islamic State militants in an attack on a Libyan oil field, the Austrian foreign ministry says.

The workers - from Austria, the Czech Republic and seven non-EU nationals - were taken alive, the statement said.

The attack on the al-Ghani field about 700km (440 miles) south-east of Tripoli happened on Friday, a Libyan army spokesman has told the BBC.

He said eight guards had been beheaded in the attack.

One other oil worker died of a heart attack after witnessing the beheadings, he added.

The foreigners worked for the oilfield management company Value Added Oilfield Services (VAOS).

"None of our employees have died or were physically harmed in the attack," VAOS said in a statement.

"We do not yet know which militia is responsible for this act and where the group has been taken to," it adds.

Authorities in the Philippines confirmed that four of the workers were their nationals, bringing the number of Filipinos missing in Libya to seven.

The group also includes workers from Austria, the Czech Republic, Bangladesh and Ghana with the nationality of one worker unknown.

The militants' long-term aim was to take over Libya's oil industry, Libyan army spokesman Ahmed al-Mesmari said, referring to a series of attacks in recent weeks.

Last week, Islamist militants were reported to be behind an attack on two oil fields in Bahi and Mabruk.

Rival militias have been fighting for control in Libya since Muammar Gaddafi was ousted in 2011.