Striking workers have shut down U.S. oil major ExxonMobil Corp.’s (NYSE: XOM) Nigeria headquarters in Lagos after more than 100 employees were fired, a labor union said Dec. 16.

Employees went on strike after ExxonMobil on Dec. 12 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.

ExxonMobil was not available for immediate comment.

Labor unions have in recent months criticized 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.