RIO DE JANEIRO, Dec 15 (Reuters) - The board of Petro Rio SA has approved a follow-on offering of approximately $250 million, the Rio de Janeiro-based oil company said in a Tuesday securities filing.

Petro Rio has made a number of significant acquisitions over the last two years, and is now Brazil's largest oil company after state-run Petroleo Brasileiro SA.

(Reporting by Gram Slattery Editing by Chris Reese)