Militants with the Islamic State of Iraq and Syria laid siege to a key oil refinery in Baiji, about 130 miles north of the Iraqi capital.
U.S. State Department spokeswoman Jen Psaki said during a regular press briefing production was shut down at the refinery for several days before the attack. The closure of the refinery, which feeds the domestic market, means Iraq may have to import some energy supplies for neighboring countries, though exports aren't limited.
"There's no impact on Iraq's crude oil exports, and we haven't seen any major disruptions in oil supplies in Iraq," she said. "But we're certainly constantly monitoring the global oil supply and demand situation."
The growing Sunni insurgency has sparked fears about the stability of global oil markets that rely in part on Iraq's supplies.
In a report Monday, auto club AAA said the conflict in Iraq was influencing U.S. retail gasoline prices. Prices at the pump normally decline in June, though AAA said the turmoil in Iraq is likely going to prevent that from happening.
ISIS took control of parts of Iraq last week. In a weekly report on petroleum, the U.S. Energy Information Administration said the United States imported on average 468,000 barrels of oil from Iraq during the week ending June 13. That was an increase of 16,000 bpd from the previous week.