During the month, exports averaged 1.1 million barrels a day, down from 1.2 million in November, the interim government said.
Shamkhi Faraj, the director general of economics and oil marketing, blamed the fall in exports on the shutdown of Iraq's largest oil refinery because of death threats aimed at truck drivers.
"The exports fell back because of the security situation," Faraj told the BBC.
On Monday, Oil Minister Ibrahim Bahr al-Uloum resigned. He was suspended for a month on Friday and replaced by Deputy Prime Minister Ahmed Chalabi.