(Corrects eighth paragraph to show that ANP criteria reflects the standards of the Society of Petroleum Engineers, not the American Petroleum Institute)
* Growth of proven reserves slows from 1 pct in 2011
* Pressure growing to increase output in existing fields
SAO PAULO, Jan 10 (Reuters) - Brazil's state-led oil company Petrobras said on Thursday its proven reserves of oil and gas grew 0.1 percent in 2012, slowing from 1 percent growth the year before as its focus shifted to improving productivity in existing fields.
The company, listed as Petroleo Brasileiro SA, said crude reserves reached 12.884 billion barrels of oil and natural gas equivalent (boe) at the end of 2012, based on the criteria of the U.S. Securities and Exchange Commission.
The increase was due to new discoveries in Brazil and the Gulf of Mexico, which were far smaller than the massive finds in Brazil's Santos and Campos basins that were incorporated into reserves in 2011. Before oil discoveries are added to a company's proven reserves, it needs to demonstrate they are technically accessible and commercially viable.
Brazil's vast offshore crude discoveries could help the nation become one of the four largest oil producers this decade. Former Petrobras Chief Executive Jose Sergio Gabrielli has said recent finds could more than double the company's reserves.
But Petrobras has struggled with slipping output in the fields it already operates, even as it spends $237 billion on a five-year investment plan, the world's largest.
Moody's Investors Service highlighted struggles to increase output and cash flow for investments when it put Petrobras on watch for a possible debt downgrade last month.
Based on the looser criteria for proven reserves used by the Brazilian petroleum regulator ANP, Petrobras said its reserves rose 0.2 percent to 16.440 billion boe, according to a filing.
The ANP bases its criteria on the standards of the Society of Petroleum Engineers, which focus on geological rather than commercial factors. (Reporting by Brad Haynes; Additional reporting by Sergio Spagnuolo and Juliana Schincariol in Rio de Janeiro; Editing by Gary Hill, Bernard Orr and Bob Burgdorfer)