"The United States has advised all of our companies, including ExxonMobil... that they run significant political and legal risks if they sign contracts with any parties in Iraq before there has been a national agreement," on how to distribute oil revenue, State Department spokeswoman Victoria Nuland said.

Last week, an Iraqi oil ministry official said Baghdad was studying its options with respect to ExxonMobil, which last month signed a deal with Kurdistan to explore six areas.

The Iraqi oil ministry official said two of those areas -- Al-Qosh and Bardarash -- are actually parts of Nineveh province that Kurdistan wants to annex into its autonomous region, a move Baghdad opposes.

Baghdad regards any contracts not signed with the central government as invalid. In the past, Iraq has banned oil companies that have signed contracts in Kurdistan from taking part in tenders or contracts for other fields.

"For many years, in fact, the United States has been urging all parties in Iraq to enact the necessary national laws that can govern the oil and gas sector," Nuland said.

"The sooner they do that, the sooner companies can -- can invest in a legally viable way."