Pirates using a two-tiered strategy have stepped up their attacks on refineries in the Amazon, including those operated by state-owned Petrobras (PBR) - Get Report , Brazilian newspaper Estado reported Wednesday. 

Smaller, faster vessels with heavy armaments force workers to surrender, then larger boats follow to siphon off the booty. The National Union of Fuel and Lubricant Distribution Companies estimates that more than 10 billion liters of fuel were stolen from the Amazon basin in 2016. 

Pirates loot an estimated $100 million a year in the Amazon. 

Petrobras shares were up 1.3% to 48.66 in premarket trading Tuesday. 

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