Islamic State (IS) insurgents took control of large parts of Iraq's north and west in a sweeping offensive that began on June 9, preventing Baghdad from exporting oil via a pipeline to Turkey and by road to Jordan.
Exports totalled 2.42 million barrels of oil per day in June, the oil ministry said, falling far short of a budgeted projection of 3.4 million bpd.
The dip adds to the woes of Iraq, which is heavily dependent on oil revenues while spending more on military equipment to battle the IS.
As OPEC's number-two oil producer, Iraq's 11 per cent of proven world reserves plays a key role on world markets and prices after violence disrupted oil exports from Syria and Libya.
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However, oil prices turned lower today, with Brent North Sea crude for delivery in September dropping 25 cents to stand at USD 107.78 in London midday deals.
US benchmark West Texas Intermediate for September fell 32 cents to USD 102.80 a barrel.
"The market is comfortably supplied with seasonal demand in Europe still slackening," said Andrey Kryuchenkov, analyst at financial group VTB Capital.
But the violence elsewhere has dimmed prospects of more foreign investment in Iraq's battered oil infrastructure.
Iraq's main oil refinery at Baiji, north of Baghdad, has been the scene of fierce battles between state forces and Islamist militants.
Western majors including BP, ExxonMobil and Shell, along with state-backed Chinese giants CNOOC and CNPC, have ploughed billions of dollars into Iraqi oil fields since 2008.