Nigeria's main militant group called off a three-month-old ceasefire in the Niger Delta on Saturday and threatened to unleash "an all-out assault" on Africa's biggest oil and gas industry.

The Movement for the Emancipation of the Niger Delta (MEND), responsible for years of attacks on oil facilities, said it could no longer trust the government to negotiate demands for greater control of the region's natural resources.

The threat of unrest comes as President Umaru Yar'Adua has been out of the country receiving medical treatment for more than two months and there is uncertainty over who is in charge of state affairs.

"It is sufficiently clear at this point in time the government of Nigeria has no intentions of considering the demands made by this group for the control of the resources and land," said the group.

"All companies related to the oil industry in the Niger Delta should prepare for an all-out onslaught against their installations and personnel," it said.

Attacks by MEND on Nigeria's oil and gas industry in the past few years have prevented the OPEC member from producing much above two-thirds of its capacity, costing it about $1 billion a month in lost revenues.

The instability has at times helped push up world oil prices.

Violence has subsided in the Niger Delta since Yar'Adua's amnesty programme last year led to thousands of militants surrendering their weapons in exchange for clemency, a monthly stipend, education and job opportunities.

MEND has been severely weakened with the departure of senior field commanders who accepted the amnesty, but oil infrastructure in the Niger Delta is extremely exposed and it takes little to launch an attack.

There have been delays to the promised monthly stipends and retraining programmes and analysts have said many former militants could return to the delta's creeks and resume attacks if they are not quickly given work and a source of income.

The slow progress in implementing the post-amnesty programme has been made worse by the absence of Yar'Adua, who left Nigeria to receive medical treatment in a hospital in Saudi Arabia more than two months ago. – Reuters