Iraq's prime minister has announced the launch of the second phase of a large-scale military operation to drive Islamic State militants out of the central Salahuddin province.

A statement from Haider al-Abadi's office late Monday said the operation, dubbed Labeik ya Rasool Allah, or "At your command, messenger of God," will aim "to liberate the whole of Salahuddin province, including the Beiji refinery," the country's largest.

Iraqi troops backed by U.S. airstrikes and Shiite and Sunni militias recaptured Salahuddin's provincial capital, Tikrit, in April, but their efforts have since stalled. Iraqi and U.S. officials have said the extreme summer heat caused setbacks.