Venezuela's oil minister, Rafael Ramirez, confirmed on Tuesday that the Organisation of Petroleum Exporting Countries would offer the extra oil for three months, and Libyan Oil Minister Fathi bin Shatwan said the organisation's output ceiling would remain unchanged at 28 million barrels a day.
Opec agreed the deal making available its remaining spare production capacity in a bid to reassure consumer countries about energy supply security, Ramirez said.
The offer applies from 1 October for three months. 
Fuel processing capacity in the United States, the world's biggest oil consumer, is running well below normal after Hurricane Katrina disabled some US Gulf refineries.