Suspected oil thieves in Nigeria have increased their attacks on Royal Dutch Shell-operated oil facilities, the company said on Friday, reporting five separate incidents already in the last three months.

Shell said suspected thieves sabotaged five oil wellheads in the oil-producing Niger Delta since Aug. 14, some resulting in fires. No production was affected since most of the oilfields were already shutdown because of insecurity in the region.

Nigeria is the world's eighth biggest exporter of crude oil but thieves take a sizeable proportion of its output by drilling into pipelines or hijacking barges loaded with oil, a type of theft known locally as bunkering.

We have growing concerns over the number of oil wellhead incidents due to apparent oil bunkering, said a Shell spokesman in Nigeria.

These incidents have severely tasked the resources of (Shell's SPDC unit).

Shell said four of its damaged oil wellheads -- Bomu 41/50, Egwa 14, Jones Creek 47 and Odidi 14 -- were under control. But its Odidi 36 oil wellhead in Delta state continues to leak three months after the suspected sabotage. SPDC is continuing efforts to recover spilled oil and gain access to the site to stop a well head spill at Odidi 36 in the western Delta, the firm said.

The incident is strongly believed to be sabotage and has resulted in a virtual well blowout. Odidi is an insecure area with difficult swamp terrain.

The company said it could not yet determine the extent of the environmental damage until it completes its investigation.

Some estimates say 100,000 barrels of crude are stolen from the Niger Delta each day, about five percent of the country's crude production and equivalent to around $7.75 million daily or $2.8 billion a year at current prices. (Reporting by Oludare Mayowa; Writing by Randy Fabi)

Crude oil Nigeria