UPDATE 1-Exxon says all its Nigeria oil output shut

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LONDON, April 28 (Reuters) - Exxon Mobil (XOM.N) said on Monday all of its oil production in Nigeria has been shut due to a workers' strike.

"Essentially all production of ExxonMobil's affiliates in Nigeria has been shut in. The affiliates will not speculate about the length of the shut-in nor how long it will take to restart production," the U.S. major said in an emailed statement.

Gloria Essien-Danner with the external affairs department of Exxon's Nigerian unit, Mobil Producing Nigeria Unlimited, said the company had declared force majeure.

"Exxon Mobil affiliates can confirm that they have given notice of force majeure as a result of the shut-in of affiliate-operated production in Nigeria," she said in a statement.

"This was caused by the withdrawal of operations services by the Petroleum and Natural Gas Senior Staff Association (PENGASSAN), the labour union."

Exxon said its affiliates in Nigeria pumped about 800,000 barrels per day in 2007.

(Reporting by Ikuko Kao)