Iraq on Monday invited foreign companies to bid for contracts developing eight of its oil and gas fields, launching a process that will let big international oil companies back into the country for the first time in more than 30 years.

Hussein Shahristani, oil minister, said the six oilfields being offered for tender were the “backbone” of Iraq’s industry, and hoped foreign investment would help lift output to 4.5m barrels per day by 2013, from 2.5m b/d today.