UPDATE 3-Nigerian oil union says will suspend nationwide strike

* Oil union called strike after members laid off

* Labour ministry, domestic oil firm held talks with union

* Action had threatened Nigerian crude output (Recasts with strike suspension)

LAGOS, Dec 18 (Reuters) - A major Nigerian oil union said on Monday it would suspend a nationwide strike on Monday after securing worker demands through dispute resolution with the government and an oil firm.

The Petroleum and Natural Gas Senior Staff Association of Nigeria (PENGASSAN), whose members are mostly managers in the upstream oil industry, began industrial action on Monday over layoffs made by a Nigerian oil and gas company.

"The strike has not been suspended for now but we are 100 percent sure it will be suspended," said Lumumba Okugbawa, the general secretary of the Petroleum and Natural Gas Senior Staff Association of Nigeria (PENGASSAN).

He said demands had been met through the mediation talks, but did not give details.

The strike has threatened to hit the country's crude production and dent exports, as was the case in December 2016 during industrial action against Exxon Mobil. But there was no immediate sign that output was affected on Monday.

Nigeria is Africa's largest crude exporter and oil sales make up two-thirds of government revenue.

PENGASSAN had not said how many workers had been involved in the industrial action, which was called after domestic oil and gas companies and marginal field operators laid off members of the union.

Marginal fields refer to discoveries made by oil majors during exploration of larger acreage but which have been left for others to develop.

(Additional reporting by Seun Sanni in Lagos, Camillus Eboh and Paul Carsten in Abuja, Kolowale Adewale in Maiduguri and Buhari Bello in Jos; Editing by Adrian Croft and Edmund Blair)