On Wednesday, a Nigerian helicopter gunship opened fire on eight barges allegedly used by smugglers to transport stolen crude oil from the creeks of the delta to tanker ships waiting offshore, a military spokesman said.

Following the raid a local militant organisation accused the army of targeting civilians and threatened violent reprisals against Shell to punish the firm for allowing its airstrip to be used for military operations.

Shell did not confirm or deny that its Osubi airstrip in Warri had been the base of the attack, but the local military task force's Mi-35P ground attack chopper was seen there Wednesday by witnesses who spoke to AFP.

"Armed intervention is always a decision for the proper authorities and not for private companies such as Shell," a Shell spokeswoman told AFP.

"As in any other part of the world, the government has a duty and an obligation to uphold the rule of law, while at the same time respecting the human rights of its people," she said.

"Any questions about military operations and their use of airstrips during their operations in the Niger Delta should be raised with the appropriate authority," she added.

Osubi, lying just north of Warri, was built by Shell as the city developed as a centre of Nigeria's oil industry. It is still managed by the Anglo-Dutch giant, but is also used by other military and civilian air operators.

The Mi-35P -- a Russian-built armoured gunship also known as a "Hind", equipped with rockets and twin 30mm cannon -- is often to be seen there.

Last month, Shell's activities in the region were targeted by an ethnic Ijaw militant group angered by the arrest of two local leaders. The group blew up a pipeline, kidnapped for foreign workers and killed 14 government soldiers.

After Wednesday's air strike, a statement from an e-mail address used by the gang demanded that Shell stop allowing Osubi be used by the military.

"This airstrip is supposed to be utilised for civilian purposes but is now apparently being utilised by the military with the consent of Shell as a staging ground for attacks on Ijaw settlements in the Niger Delta," it said.

The group warned "we are very well capable of shooting down aircraft landing and taking off" from Osubi and might do so if "the use of this privately owned civilian airstrip for military operations is not discontinued".

Nigeria is Africa's biggest oil exporter, producing around 2.6 million barrels per day, but most of its 130 million people live in grinding poverty and there is much resentment in the delta of government and the oil firms.