LAGOS, Dec 16 (Reuters) - Striking workers have shut down U.S. oil major Exxon Mobil Corp's Nigeria headquarters in Lagos after more than 100 employees were fired, a labour union said on Friday.

Employees went on strike after Exxon Mobil on Monday began to issue letters to around 150 Nigerian workers without informing the Petroleum and Natural Gas Senior Staff Association of Nigeria (PENGASSAN), Lumumba Okugbawa, the union's acting general secretary, said.

Exxon Mobil was not available for immediate comment.

Labour unions have in recent months criticised oil firms for laying off workers. The industry has been hit by low crude prices and a wave of militant attacks in Nigeria's oil hub, the Niger Delta, choking production capability.

The U.S. oil company had wanted to start making employees redundant since last year, and was in talks with PENGASSAN, which lobbied for voluntary redundancies, Okugbawa said.

"Suddenly this week the management began an involuntary redundancy exercise sacking workers whether they want to leave or not thus repudiating our agreement with them," he said.

(Reporting by Anamesere Igboeroteonwu; Additional reporting by Libby George in London; Writing by Paul Carsten; editing by Susan Thomas)