Ethnic militants fighting for local control over Nigeria's oil wealth have threatened to attack the police in Rivers state, located in the Niger Delta.  The threat followed the killing last week of three militants by the police.

Security has been stepped up in Nigeria's main oil city, Port Harcourt, as the city braces itself for fresh attacks by the Movement for the Emancipation of the Niger Delta, known as MEND.

The group has staged a series of attacks this year that have killed an unknown number of security forces. MEND, a loose coalition of militant groups, has vowed swift revenge for what it alleged were the summary executions of three of its so-called freedom fighters.

The police confirmed three people had been killed, but denied they were executed. The police said they were armed robbers who were killed in an ambush.

Port Harcourt, the capital of Rivers state, has suffered attacks from militants groups in recent weeks. Kelvin Ebiri, a Port Harcourt resident, says there is now a pervasive sense of insecurity in the southern oil city.

"The fear of insecurity in Port Harcourt is very real," he said. "There is a sense of insecurity here, no doubt about it. Port Harcourt night life used to be a thriving one but now because of the insecurity, you hardly find people out. Like in my office area, once it is 6; 6.30; 7 p.m you hardly find taxis along the road."

The police, however,are playing down on the threat of further violence and the prevailing tension. State police chief, Samuel Agbetuyi, says security forces are battle-ready for the insurgents.

"If a section of Nigeria is threatening they will kill the others, then we will wait now, for them. But we are not fighting each other. Nobody is fighting any other person,"said Agbetuyi.

MEND has waged a series of attacks and kidnappings since January against oil industry targets in the region, leading to an estimated 20 percent drop in Nigeria's oil production. Nigeria is Africa's largest oil producer and the world's sixth exporter.

Analysts say the current wave of violence could seriously hurt the oil-and-gas industry.