By Alex Last
BBC News, Lagos
Nigerian forces have raided a poor district in the city of Port Harcourt in the Niger Delta, officials say.
Militant attacks have cut Nigeria's oil production by 25%
Soldiers were going from house to house looking for members of armed gangs, according to reports in the area.
On Tuesday Nigerian President Olusegun Obasanjo ordered a crackdown on armed groups after a spate of kidnappings of foreign oil workers.
The abductions and attacks on oil facilities have cut Nigeria's oil production by 25%.
Residents in Port Harcourt described hearing helicopters flying overhead and then the sound of sporadic gunfire as troops entered a poor slum area on the city's waterfront.
Security sources say this is the start of the government's attempt to crack down on armed groups in the Niger Delta who have been responsible for six separate kidnappings of foreign oil workers in the space of two weeks.
The president announced the new, more aggressive policy on Tuesday.
For the last few days residents have been bracing themselves for the military's move.
But privately some diplomats and security experts have been concerned that a heavy-handed approach would exacerbate an already volatile region and endanger the lives of both civilians and the hostages still being held.
The Delta is awash with weapons. Youth unemployment and poverty are high, so many young men have joined armed groups.
Some are involved in kidnappings for ransom, some in the lucrative trade in smuggled crude oil.
Some are used by militant leaders to launch attacks on the oil industry as part of their campaign for local control of the oil wealth.
But no matter how effective the military operation is, it will not solve the underlying causes of the violence.