Italian oil major Eni fears its largest oilfield in Libya, known as Elephant, may be in ruins, which could dash hopes of a speedy return of Libyan supplies to global markets after months of war.

"One volunteer went with a squad of 10 rebel fighters, who escorted him on a two-to-three-hour survey in which he took pictures of the mess," ENI's Libyan operations manager, Mustafa Abougfeefa, said in an interview.

The field, which pumped 130,000 barrels of oil per day before the war, was found in ruins, with its airport completely destroyed along with crucial monitors and key electronic structures, he said.

"We cannot promise the field will start producing before the end of the year. Gaddafi's militia destroyed everything," Abougfeefa said.

The process could take even longer because the area remains a hotbed where clashes between rebel fighters and Gaddafi troops continue.

The scale of damage discovered at El Feel, known as the Elephant field for its size, was a stark contrast to the conditions of Eni's smaller Abu Atiffel and Wafa fields, which have already started production at a combined rate of 76,000 bpd, around 30 per cent below their pre-war output levels.