The strike at ExxonMobil's Nigeria affiliate entered its fourth day Sunday with the shutdown of nearly all of the company's 800,000 barrels per day output. Gilbert da Costa in Abuja reports talks to resolve the dispute have been constrained by pre-conditions set by the company and union.

Mobil oil company head office building in Lagos, Nigeria

Mobil Producing Nigeria, a unit of U.S. oil giant ExxonMobil, says the workers should call off the strike to facilitate the resumption of negotiations.

The Petroleum and Gas Senior Staff Association, the union representing the aggrieved white-collar workers, says it is ready to resume talks, but without preconditions, which means the strike would continue.

National president of the oil workers union, Peter Esele, insists the company's intransigence could complicate efforts to resolve the dispute.

"They are so used to the fact that we will always say we will shut down and we do not do it. So, I think at the end of the day, they became complacent," said Esele.

Senior oil workers began the strike Thursday following the collapse of crisis talks with the company over demands for higher wages and working conditions.

The Nigerian government is trying to mediate the dispute, and has scheduled a meeting for Monday, in Abuja. Union officials are conferring with government representatives before that meeting.

More than half Nigeria's oil output of 2.5 million barrels per day has been cut due to unrest in the oil producing region and the ongoing strike.

Mobil Producing Nigeria is the second-largest oil company in Nigeria after Royal Dutch Shell. Oil companies in Nigeria have seen output drop because of militant attacks in the Niger Delta.

The main militant group in the southern oil region sabotaged a pipeline belonging to Shell on Friday, as the strike paralyzed ExxonMobil's production in Africa's biggest producer.