Nigerian militants said on Tuesday they had attacked a pipeline feeding an oil refinery in the Niger Delta, underscoring the vulnerability of energy infrastructure despite recent military successes in the region.
The Movement for the Emancipation of the Niger Delta (MEND) said in a statement emailed to media that its fighters attacked the pipeline to the Warri refinery on Sunday. It said it would attack more pipelines in the coming days.
State-run oil firm NNPC has said it is repairing damage to the Warri-Escravos oil pipeline but made no further comment on Tuesday. The Warri refinery has a capacity of around 125,000 barrels per day (bpd).
"(These attacks) are a reminder to the Nigerian government of the futility of wasting the nation's resources in combating militancy without addressing the underlying causes of agitation in the Niger Delta," MEND said.
The authorities have claimed significant victories over MEND in recent days, freeing 19 hostages held by the group last Wednesday and then arresting the commander responsible and more than 60 of his followers.
The successes were a boost for President Goodluck Jonathan, who is the first head of state from the region and who brokered an amnesty there last year. A resurgence of violence could undermine his credibility ahead of elections next April.
The negotiations to release the hostages involved former MEND commanders who accepted last year's amnesty.
Security experts said co-operation between the former militants and the armed forces was a significant new development which could help prevent MEND from rising to prominence again. But they also warned it was impossible to fully guard against attacks by small groups of gunmen on the industry.
EXPOSED PIPELINES, RIGS
Previous campaigns by MEND fighters have knocked out a significant chunk of Nigeria's oil production, currently averaging around 2.2 million barrels per day (bpd), and cost it as much as $1 billion a month in lost revenues.
Oil infrastructure in the delta, a network of thousands of shallow creeks opening into the Gulf of Guinea, is extremely exposed, with thousands of kilometres (miles) of pipeline passing through remote and thickly forested terrain.
Disputes between local communities and oil firms are common, and attacking a pipeline and shutting down production requires little more than simple home-made explosives.
Shell declared force majeure on its Bonny Light oil exports on Friday after a pipeline was damaged, freeing it from shipment obligations, though there was no immediate evidence of links to militant activity.
It is also extremely difficult to protect offshore platforms such as those operated by Exxon Mobil and British-based Afren, from where 15 of the 19 hostages were kidnapped.
For such raids, the militants use open craft too small to be detected by radar. They are even able to operate far offshore by using a "mother ship", a larger vessel which supplies the speedboats with fuel and food, security experts say.