A trade union for workers in Nigeria's oil and gas industry has said it will shut down all production from Sunday if the government fails to reverse a decision to end fuel subsidies. Nationwide strikes over the decision, which has doubled petrol prices, are into their fourth day. Nigeria is Africa's biggest oil exporter, and the unrest has helped raise world prices. The country has little capacity to refine crude oil. Many poverty-stricken Nigerians see subsidised fuel as the only benefit they get from their country's oil wealth. There have been demonstrations across Nigeria since the decision was announced on 1 January. Petrol prices and transport fares have doubled, and since the strikes began on Monday, tens of thousands of people have taken to the streets around the country to protest. There have been big protests in the largest city, Lagos and also in Kano, the main city in the north. On Wednesday, the authorities imposed a 24-hour curfew in Niger state after unrest. During the violent protests in the state capital Minna, hundreds of rioters set fire to government and political party offices and also targeted the homes of local politicians. Partial curfews are also in place in the states of Kano, Zamfara, Borno and Oyo. 'Bitter option' The Petroleum and Natural Gas Senior Staff Association of Nigeria (Pengassan) said it had put all production platforms on red alert in advance of the shutdown. "We are hereby notifying the Federal Government of Nigeria... that Pengassan shall be forced to go ahead and apply the bitter option of ordering the systematic shutting down of oil and gas production with effect from... 0000 hours of Sunday 15 January," it said in a statement. Union president Babatunde Ogun said workers had already stopped sending production reports to the government. Later AFP news agency reported that union leaders were going to meet President Goodluck Jonathan for the first time since the strike started. Some analysts have suggested that the unions would not be able to halt production, but the authorities have expressed concern about the prospect. "If they go ahead to carry out their threat, that action will worsen our economic problem which the government is trying to solve, this is why the government is calling on labour and the civil society to come for dialogue," Information Minister Labaran Maku told Reuters news agency on Wednesday. President Jonathan says ending the subsidy will save the government $8bn (£5.2bn) a year, which will be put into public services. Nigeria is a top supplier of crude oil to the United States and European Union, producing about 2.4 billion barrels a day. The industrial unrest in Nigeria - and the increasing threat of an embargo on Iranian fuel exports - have caused international oil prices to rise. One of the main international benchmarks, Brent crude, has risen by more than $1 to $113 a barrel. The unrest comes as Mr Jonathan tries to tackle a new wave of violence from Islamist insurgents.