Iraq reiterated that only the Ministry of Oil was authorized to carry out sales - an apparent swipe at the Kurdish region - and buyers who dealt with other parties risked punishment such as sanctions.
Islamic State has seized five oil fields in the north since it arrived in June from Syria, prompting Iraq's U.S.-funded soldiers to flee.
On another note, Iraq's outgoing foreign minister said Kurdish ministers who suspended their participation in the government of outgoing Prime Minister Nuri al-Maliki have rejoined the administration.
"I am back in Baghdad as foreign minister," Hoshiyar Zebari, a Kurd, told Reuters.
Maliki infuriated Kurdish leaders by accusing them of harboring terrorists after Islamic State militants swept through northern Iraq in June.