An oil leak has caused a production halt at a Shell facility off Nigeria capable of producing 200,000 barrels per day, the company said Wednesday, adding that \"less than 40,000 barrels\" have spilled. \"Shell Nigeria Exploration and Production Company can confirm it is responding to an oil leak at the 200,000 barrel-per-day capacity Bonga deepwater facility, approximately 120 kilometres off the Nigeria coast,\" a statement said. \"Early indications show that less than 40,000 barrels of oil have leaked in total.\" A spokesman confirmed production had been shut there. Shell said the leak occurred Tuesday during a transfer of crude to a waiting tanker off Nigeria, Africa\'s largest oil producer and an OPEC member which has been producing between 2.0 and 2.4 million barrels per day in recent months. \"An export line linking the (production and storage vessel) to the tanker has been identified as the likely source, and has been closed and de-pressurised, halting the flow of oil,\" it said, adding no one was injured. Shell Nigeria head Mutiu Sunmonu apologised for the leak in the statement and said the company was working to quickly clean it up. \"We are sorry this leak has happened,\" he said. \"As soon as we became aware of it, we stopped the flow of oil and mobilised our own resources, as well as industry expertise, to ensure its effects are minimised. It is important to stress that this was not a well control incident of any sort, and to make clear that no-one has been injured. \"Our focus now is on a speedy and effective clean-up.\" It said Nigerian authorities had been notified and Shell\'s emergency response team had been activated. A spokesman could not give more specific numbers on the amount of oil spilled, saying an investigation was underway. In April, Shell had resumed production at Bonga after shutting the facility in February for maintenance. It was also hit by a militant attack in 2008. Scores of oil spills have occurred in Nigeria, but most have been at onshore sites, particularly due to pipeline sabotage aimed at stealing crude to sell on the black market as well as militant attacks. Activists say Shell and other companies have not done enough to prevent oil leaks at such sites. The UN released a report in August saying decades of oil spills in the Nigerian region of Ogoniland may require the biggest clean-up ever undertaken, with communities dependent upon farmers and fishermen left ravaged. Shell\'s Nigerian joint venture was taken to task in the UN report, which said its procedures for control and maintenance of infrastructure had not been followed. Oil spills had also not been sufficiently cleaned, according to the report. Amnesty International has estimated that if all types of oil pollution in the vast Niger Delta -- the country\'s oil-producing region -- are added up over the past half-century, it would be \"on par with the Exxon Valdez every year over the last 50 years.\" Production in Nigeria has rebounded in recent years following a 2009 amnesty deal for militants in the delta which led to a sharp decline in unrest there. Militants claiming to be fighting for a fairer distribution of oil revenue as well as criminal gangs had carried out years of attacks and kidnappings in the region. According to OPEC\'s 2010 figures, Nigeria overtook Iran as the cartel\'s second-largest crude exporter.