The move comes after a year-long stand-off between the American energy firm and Iraq's central government, which has opposed contracts such as the one signed by Exxon with the autonomous northern Kurdistan region because it was not explicitly approved by Baghdad.

"ExxonMobil wants to sell all or part of its share to other companies and will exit," said Abdul Mehdi al-Amidi, head of the Iraqi oil ministry's contracting and licensing department.

"There is an unclear message from ExxonMobil to the (state-owned) South Oil Company in which Exxon said that there are negotiations ongoing with other companies" to sell its stake in the West Qurna-1 field.

Asked if Iraq had issued any response to Exxon's message, Amidi declined to answer.

In January 2010, Iraq completed a deal with Exxon and Anglo-Dutch giant Shell to develop production at West Qurna-1. With reserves of about 8.5 billion barrels, it is the country's second largest oilfield.

In October 2011, however, Exxon signed an oil exploration deal with the Kurdistan region in northern Iraq covering six areas, including two that are said to be outside the region and claimed by both Baghdad and Kurdistan.

The central government says all oil contracts must go through Baghdad and regards any that do not so as illegal.