(Adds Exxon comment)
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 said it was aware of "a unilateral industrial action" by the union without commenting on the reported sackings.
"We will continue to monitor the situation," the firm said. "We remain committed to safety of our personnel, protection of our facilities and the environment."
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 and Mark Potter)