Tensions between the Kurdish regional capital Arbil and Baghdad have increased since the start-up of a new KRG-controlled pipeline to the Turkish port of Ceyhan in May.

Iraq's autonomous Kurdish region has hit back at Baghdad over independent oil exports, a letter from the Kurdistan Regional Government (KRG) showed, threatening to counter sue the central government for trying to block its sales.

The strongly worded letter shows growing confidence from the Kurdish capital Arbil in the long-running oil sales dispute, as Baghdad struggles to regain control of swathes of territory lost to ISIL.

The letter, addressed to Iraqi Oil Minister Abdul Karim Luaibi from KRG Natural Resource Minister Ashti Hawrami, said the Kurds would pursue legal action by the middle of this month if Baghdad does not stop its "interference".

"(The) KRG will bring civil, and where necessary, criminal proceedings against your Ministry and any person, foreign advisor, or any entity conspiring with your Ministry in any form," Hawrami wrote, in the letter dated June 29 and carried on a KRG website. He did not specify a court for the action.

The autonomous Kurdish region has been trying to establish greater financial independence from Baghdad by selling its own oil production directly on international markets. It has largely been spared the violence affecting much of Iraq.