Nigeria's troubled oil-rich Niger Delta has witnessed an upsurge in violence, with fighting between militants and Nigerian troops, in which militants claim at least a dozen soldiers were killed. The government's handling of the crisis has come in for some criticism.

The renewed violence, which comes after a month of relative calm, underlines the oil-rich southern Delta region's volatility.

Seven foreign oil workers were seized from a residential facility early this week. However, 25 oil workers abducted on Monday have been released.

Militants in the region have crippled the oil industry with a series of kidnappings and attacks on oil facilities, disrupting production in Africa's largest oil exporter, partially shutting down production.

The Nigerian government has launched several crackdowns on the violence, with limited success.

Nigeria's two main oil unions staged a warning strike in September to draw attention to the spiraling violence and threats to oil workers in the Delta.

Peter Esele, president of the Petroleum and Natural Gas Senior Staff Association, says the Delta violence underscores a critical political problem in Nigeria.

"It is very sad that we have to go back, nobody seems to be moving, and that is the saddest part of the whole thing," he said. "Failure of government, failure in terms of leadership, lack of confidence in the government, also by the followership. There is a whole gap of disconnect between the governed and the government, and that is a big gulf we are trying to fill."

Recent gun battles between militants and troops have forced Shell oil company to shut down some of its production facilities, adding to about 500,000 barrels a day shut since February in the Delta.

Esele says the growing violence may compel oil companies, such as Wilbros, to pull out of the area. He warns that could trigger more socio-economic problems.

"Wilbros wants to close shop in the Niger Delta, and we have a series of companies trying to get out of the Delta. And we are fighting for a better life in the Niger Delta, and we have companies moving out. If these companies are pulling out, jobs will be lost and the frustration will increase," said Esele.

The Niger Delta, which is the source of all of Nigeria's crude oil, has a long history of violence. Agitation for a greater share of the country's huge oil wealth, job opportunities and the provision of basic amenities are driving the unrest.