(Updates with Shell reaction)
LIBREVILLE, Jan 12 (Reuters) - Royal Dutch Shell workers in Gabon began an "unlimited" strike on Thursday at all the company's operations in the Central African OPEC member country, the workers' union wrote in a letter to employees.
Shell is in advanced talks to sell its Gabon assets, which one source estimated could be worth $700 million, leaving workers worried about layoffs or being moved to new locations, the union said.
The national union of petroleum employees (ONEP) said the strike "will cover all of Shell Gabon's operations (Libreville, Port-Gentil, Gamba Rabi, Koula and Toucan)".
Shell, which produces about 55,000 barrels of oil per day in Gabon, confirmed the announcement of the strike but declined to say if it had had an impact on production. It operates the Gamba export terminal through which an additional 20,000 barrels from other producers are also exported.
"Our local management team in Gabon is engaged in mediation with the Ministry of Labour and the union ONEP relating to impacts on employees from our advanced negotiations with a potential buyer," a Shell spokesman said.
The union demanded in December that all Shell Gabon employees be transferred to whichever company takes on Shell's assets, and that no redundancies were made for economic reasons within five years of the deal.
Gabon is Africa's fourth largest oil producer with an output of around 220,000 barrels per day dominated by international oil majors Total and Shell. (Reporting by Wilfried Obangome; additional reporting by Ron Bousso in London; Writing by Edward McAllister; Editing by Joe Bavier and Alison Williams)