MONROVIA, Nov 19 (Reuters) - The United Nations peacekeeping mission in Liberia (UNMIL) tightened security on Monday at the country's second-largest rubber plantation after its Belgian manager was shot dead at the weekend, UNMIL said in a statement.

Michael Bruno was killed on Saturday while inspecting an area earmarked for the controversial expansion of the Dutch-owned Liberia Agriculture Company (LAC) plantation in the northern region of Buchanan.

U.N. officials and Liberian police are investigating the incident and have made three arrests, deputy Information Minister Gabriel Williams told Reuters.

The shooting came just three weeks after the LAC signed a deal with the government for the $22 million expansion of the plantation, which many local residents oppose.

Three men have also been arrested in connection with the hijacking of an LAC vehicle carrying $150,000 to pay workers' salaries, Williams said.

Rubber, tapped from trees on vast jungle plantations, remains one of Liberia's main exports in the wake of a devastating 14-year civil war which dragged the once-prosperous West African country into poverty and killed 200,000 people. (Reporting by Alphonso Toweh, Editing by Daniel Flynn)

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