On October 18, Kurdish authorities inked a deal with ExxonMobil for it to explore six areas in Kurdistan, but Baghdad regards as invalid any contracts not signed with the central government.

"We sent a letter to (Prime Minister) Mr Nuri al-Maliki to inform him of the matter, and we informed him that we will tell him all the developments, and he did not object," Kurdish president Massud Barzani said at a speech in the northern city of Dohuk.

"We sent him a letter to inform him of the details when it came time to sign the contract, and he said, 'good', but in the end, they protested and said it was not in accordance with the constitution."

Barzani also insisted that the contract complied with Iraq's constitution.

Maliki told AFP on December 15 that ExxonMobil had promised to reconsider the exploration deal, but did not elaborate.

The Kurdistan contract potentially puts an Exxon contract with the Iraqi government in jeopardy.

In January 2010, Iraq's oil ministry completed a deal with ExxonMobil and Anglo-Dutch giant Shell to develop production at West Qurna-1, which with reserves of about 8.5 billion barrels is the country's second-biggest field.