The gang shot dead eight policemen and a civilian worker in the latest attack on Africa’s biggest oil industry.
This latest raid was on a site known as the Agip Industrial Area in the southern Nigerian city of Port Harcourt, a large complex of offices, workshops and jetties run by the Italian energy giant.
"The installation was attacked between about 2.00 pm (local time) and 2.30 pm. The attackers came over the water and attacked the section of the base which houses a bank," said Maurizio Bungaro, Italy's consul general in Nigeria.
"The attackers killed nine people in all, eight police officers and a Nigerian employee. There were no dead among the assailants. The attack was well organised and they were able to take their loot and go," he said.
Local resident Damka Pueba told AFP she heard gunfire coming from the industrial area.
"One of the staff came out, she was crying. She said some boys came in speedboats and got into the company and just started shooting," Ms Pueba said. Citing witnesses Ms Pueba said the gunmen were wearing camouflage fatigues and military-style berets.
Agip's parent company ENI confirmed the death toll and said in a statement that an undetermined number of people were injured in the "exchange of fire".
"ENI has temporarily evacuated the installation and the situation is for the moment under control," the firm said.
Militia raids
The Port Harcourt shootings came shortly after ethnic separatists threatened to step up attacks on the oil industry.
A series of bloody incidents in recent months have rocked Nigeria, the world's sixth largest oil exporter, which produces 2.6 million barrels of crude per day.
The Niger Delta swamps around Port Harcourt are home to several well-armed illegal militias, known for piracy and a violent rebellion for ethnic Ijaw independence.
It’s unknown at this stage whether the gunmen who raided Agip are linked to a separatist militia which in the past three weeks was responsible for blowing up a major oil pipeline, killing 14 soldiers and kidnapping four foreign oil workers.
On January 11, rebels stormed an oil industry supply ship operating off the Niger Delta, kidnapping its American skipper, a British security expert, a Honduran engineer and a Bulgarian oil worker.
President Olusegun Obasanjo has set up a panel to find a "political solution" to the crisis and secure the hostages' safe release. But in an email to AFP, a spokesman for the gang rejected speculation that release was imminent.
"We are seeking to capture more rather than thinking of setting them free. Be assured however that the hostages will not taste freedom for as long as the
Nigerian government holds any of our citizens," the spokesman said.
The gang has demanded the release of two Ijaw leaders, Diepreye Alamieyeseigha and Mujahid Dokubo Asari in return for the hostages who it says are being hidden in creeks and mangrove forests.
Alamieyeseigha was Nigeria's only Ijaw state governor and was arrested on suspicion of misappropriating hundreds of millions of dollars.
He appeared in court in Lagos on Tuesday on corruption charges.