(Rewrites to focus on offer price, adds comment and background)
By Nicolas Misculin and Jeb Blount
BUENOS AIRES/RIO DE JANEIRO, March 2 (Reuters) - Argentina's Pampa Energia SA offered Brazilian state-run oil company Petroleo Brasileiro SA about $1.2 billion to buy its 67.2 percent stake in Petrobras Argentina SA, a source with direct knowledge of the bid said on Wednesday.
The board of Petrobras, as the Brazilian company is known, has approved exclusive talks with Pampa for up to 60 days. The discussions were announced earlier on Wednesday in a Brazilian securities filing.
Petrobras Argentina is among the four largest producers of oil and gas in the South American country and has extensive downstream operations, including refining, petrochemicals and electricity generation.
A final deal should be complete within two months, said the source, who requested anonymity because a final accord has not been signed.
"There is an agreement on the figure," the source said. "I don't see anything cumbersome to work out. Now it's the work of lawyers."
Petrobras preferred shares rose 2.82 percent in Sao Paulo in afternoon trading to 5.46 reais, on track for its highest close in seven weeks. Pampa fell 3.94 percent, in Buenos Aires, its biggest one-day drop in six weeks, and Petrobras Argentina rose 2.55 percent.
The proposed transaction comes at a key time for Petrobras and Pampa. If completed, the sale will be one of the first major deals in Petrobras' eight-month old plan to sell $15.1 billion of assets by the end of 2016.
Petrobras Chief Executive Officer Aldemir Bendine has said the company needs to sell the assets to shore up its troubled finances and pay debt of about $130 billion, the world oil industry's largest.
For its part, Pampa would widen its range of energy assets, including natural gas and oil rights. It is the largest integrated power company in Argentina, generating about 8 percent of the nation's electricity.
The talks come only months after the election of Argentine President Mauricio Macri, who promised to open the country to international trade and investment after more than a decade of increasing state control. This has raised expectations that investment in Argentina's overstretched and underdeveloped energy industry will increase.
Pampa controls Transener, which operates Argentina's largest high-tension power transmission line, and also has a stake in Edenor, the nation's largest power distributor.
In a separate filing, Petrobras said its board had also approved a competitive process to sell rights to a group of onshore oil fields, and related assets, in Brazil.
(Additional reporting by Marcelo Teixeira; Brad Haynes, and Priscila Jordao in Sao Paulo; Writing by Jeb Blount; Editing by W Simon and Lisa Von Ahn)