About 61,000 barrels of crude of oil giant Anglo-Dutch Shell were spilled in Nigeria last year, mostly due to the activities of Niger Delta armed militants, company document said on Wednesday.

"In 2008, saboteurs spilled almost 48,000 barrels of oil in 140 incidents... About 40,000 barrels of this was the result of armed gangs blowing up pipelines with explosives in 10 separate incidents," it said.

Shell's Nigerian subsidiary, the Shell Petroleum Development Company (SPDC), said that the volume of spill became significantly greater because local communities frequently delayed the company's officials from stopping the leak and start the clean-up.

"According to our own estimations, 13,000 extra barrels of oil were spilled in 2008 as a result," it added.

Local communities in the oil-rich region have often accused foreign oil companies, especially Shell, of causing environmental pollution and degradation without paying adequate compensation.

Shell was in 1994 forced to stop operation in Ogoniland -- a major community in the region -- following a violent campaign against the company.

SPDC, the largest private-sector oil and gas company in Nigeria, has a network of more than 6,000 kilometres (3,750 miles) of flowlines and pipelines and 90 oilfield in volatile Niger Delta.

It also has 1,000 oil wells, 72 flowstations (pumping stations), 10 gas plants and two major oil export terminals -- in Bonny and Forcados.

There were almost 90 incidents of crude oil theft, locally known as bunkering, from SPDC facilities last year, during which incidents of "malicious damage" and pipeline theft increased by 48 percent, it also said.

Between 2006 and 2008, armed militants kidnapped 133 SPDC workers and contractors.

Five people working for SPDC joint venture were killed in assaults and kidnappings in the same period.

Shell ventures produced an average of more than 850,000 barrels of oil equivalent per day last year, about 39 percent of the country's total oil and gas production, the document said.