Iraq's oil exports reached their highest level in decades in December, the oil ministry's spokesman said on Saturday, but vital revenues were being hit by the plummeting prices of crude.

Iraq exported 91.141 million barrels of oil in December for an average of 2.94 million barrels per day, the highest daily average since 1980, Assem Jihad told AFP, citing initial figures.

December exports were far and away the highest in 2014, eclipsing the previous best month by more than 11 million barrels, according to ministry figures.

But revenues were $5.247 billion, only up slightly on November and much lower than the early months of 2014 because of sharply declining oil prices.

Jihad said prices averaged $57 a barrel in December, down from $100 or more during the first half of the year.

The average December price was even lower than the $60 per barrel used to calculate Iraq's 2015 budget, which already includes a more than $19 billion deficit.

Oil producing cartel "OPEC must move to address this issue," Jihad said.

"Prices are falling now to abnormal levels, and it is up to the organisation to move instead of watching prices collapse to illogical levels," he said, adding that this was his personal opinion rather than that of the ministry.

The dramatic decline in oil prices poses a major problem for Iraq, which depends on oil revenues for the vast majority of government funds.

The oil ministry said in November that Baghdad lost more than 27 percent of its projected revenues for 2014 because of the fall in prices.

It comes at a time when expenditures are higher than usual, with the country battling to regain ground from the Islamic State jihadist group, which spearheaded a sweeping offensive that overran swathes of territory in June.