In the letter dated on Monday and seen by Reuters, ONEP also demanded new bonuses for employees, increased career advancement opportunities and a reduction in the number of foreign workers.

A Total spokeswoman said that Total Gabon had fully consulted workers' representatives about recent changes in the company's strategy and that discussions were continuing to resolve the dispute.

The strike would affect all of Total's facilities in Gabon, including in the capital Libreville and oil hub of Port Gentil.

The threat of strike action follows the company's failure to respond to the union's earlier demands made in May, the letter said.

The Central African nation produces about 200,000 barrels per day (bpd) of crude, according to the U.S. Energy Information Administration, but output from the OPEC member's ageing fields has plummeted from a 1997 peak of 370,000 bpd.

A steep fall in oil prices in 2014 and 2015 cut much needed revenue and forced companies to lay off thousands of oil workers.

Total produces about 54,000 barrels of oil equivalent per day in Gabon.

(Reporting by Gerauds Wilfried Obangome' additional reporting by Bate Felix; Writing by Aaron Ross; Editing by Edward McAllister, Edmund Blair and David Stamp)