(Adds concessions offered in paragraphs 7-9)

By Camillus Eboh

ABUJA, June 18 (Reuters) - Nigerian unions will start an indefinite general strike in Africa's top oil producer on Wednesday to protest against rising prices and privatisations, unions said.

The protest call, which poses an immediate challenge to newly inaugurated President Umaru Yar'Adua, came after violence flared in the oil-producing Niger Delta.

Armed militants stormed two Western oil facilities in the remote swamps of the delta and halted 82,000 barrels a day of crude production.

The Nigeria Labour Congress (NLC) and Trade Union Congress called the strike for Wednesday after the deadline passed on an ultimatum to the government to reverse a fuel price increase, revoke a doubling of value-added tax, cancel the sale of two oil refineries and backdate a public sector pay rise to January.

"With effect from Wednesday 20 June 2007, an indefinite general strike and mass protests by Nigerians will commence," said NLC President Abdulwahed Omar.

"All offices, ports, banks, petrol stations and business premises will be shut down. All schools, airports, official and semi-official business premises will be closed," he added.

LAST MINUTE CONCESSIONS

In talks with the unions after the strike call, the government offered a series of concessions in an attempt to avert the action, a presidency source said.

The government offered to take back half of the fuel price increase and revert value-added tax to five percent. It also agreed to the pay rise from Jan. 1 and told the unions to engage the privatisation agency on the sale of the two refineries, the source added.

Senior union officials at the talks declined to talk to the press, but one said the offer was not enough to avert the strike. The two sides could resume talks on Tuesday, he added.

Yar'Adua, who has yet to appoint a cabinet, inherited the controversial measures from his predecessor. Ex-president Olusegun Obasanjo carried out the price hikes and refinery sales just days before leaving office on May 29.

Union leaders said the strike would be total, and would include production and shipments from the world's eighth largest exporter of crude, although there was some uncertainty over how quickly this would happen.

Previous strikes in Nigeria have had a limited impact on oil operations, because they tend to build strength slowly and are normally resolved within a few days.

The oilfield invasions in the Niger Delta were a setback to a nascent peace initiative between Yar'Adua and militants who have crippled Nigerian oil output over the past 18 months.

Italian company Eni <ENI.MI> said 27 people were still being held at its Ogbainbiri flow station on Monday after Sunday's invasion, while U.S. giant Chevron <CVX.N> said attackers left its nearby Abiteye plant soon after invading it on Sunday.

A prominent militant leader, Mujahid Dokubo-Asari, was released on bail last week, meeting a key demand of armed groups who have led a campaign of kidnapping of foreign workers and bombings of oil facilities.

Militants have released about 30 hostages since Yar'Adua's inauguration on May 29, and various groups have called a truce to allow dialogue to start. But the situation in the southern wetlands region is still tense and there are at least a dozen other hostages still being held by other groups.

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