Iran has been systematically plundering large amounts of oil from southern Iraq for years, a theft on a grand scale that's helping the Islamic Republic withstand sanctions aimed at throttling its oil exports, a U.S. security consultancy says.

"Tehran has built a complex oil smuggling network that allows it to bring in significant revenues from southern Iraq's oil production," observed Stratfor, which has its offices in Texas.

These funds, worth around $20 million a day, enable Iran, which is predominantly Shiite Muslim, "to maintain its influence in the region while preparing to defend itself from stringent sanctions against its oil exports," Stratfor said.

Stratfor said that roughly 10 percent of the oil produced in the Basra region of Shiite-dominated southern Iraq, where two-thirds of Iraq's oil reserves lie, "is smuggled, most of which ends up in Iran for export."

Iran is increasingly feeling the impact of the sanctions regime, intensified by the United Nations in June 2010 because Tehran refuses to abandon its contentious nuclear program.

The United States and the European Union have recently tightened sanctions, with the aim of choking Iran's vital exports which produce some 80 percent of state revenue.

Iran has long coveted Iraq's oilfields, which hold known reserves of 115 billion barrels. Industry officials say untapped fields could contain as much again.

During the 1980-88 war, Iranian forces seized the big Majnoon fields along Iraq's eastern border. Iranian troops periodically occupy disputed fields inside Iraq. The last incident, in December 2009, involved the Fakka field in southeastern Maysan province with Iranian troops holding a well for several days. Fakka contains an estimated 1.5 billion barrels of oil.

These border disputes became more common at that time as Baghdad was auctioning off development rights for Iraqi fields to foreign oil companies. Those companies, including Exxon Mobil of the United States, have sharply boosted production at Iraq's major fields in the south, making them even more attractive to Iran.

Iran has long sought to dominate Iraq, its ancient enemy, particularly since the Americans toppled Saddam Hussein's regime in 2003. Tehran's influence sharply increased when the United States completed its military withdrawal in December.

"Through a web of alliances, including Iraqi politicians, unions, oil syndicates and militias, Iran is already well-positioned to extract oil revenues from southern Iraq via informal channels," Stratfor said.

"This allows Iran to draw from a large stash of resources to maintain its regional influence while insulating itself against intensifying sanctions targets Iranian oil exports …

"Iran's geopolitical ambition is to make southern Iraq an extension of itself, thereby allowing Iran to vastly, albeit indirectly, increase its share in the global energy market," Stratfor noted.

"Ideally, Iran would like to see southern Iraq evolve into a single, Shiite-dominated state within an Iraqi federation that would serve as an Iranian satellite …

"When it comes to critical policy decisions on … accepting major military assistance from the United States, Iran has ample financial might from the oil revenues it earns through smuggling to influence the minds of Iraqi decision-makers," Stratfor said.

The Iranians are also pursuing more legitimate efforts to get their hands on Iraq's oil.

In 2011, Tehran decided to give production priority to the shared fields along the poorly demarcated border that were being depleted by Iraq.

Last Saturday, Tehran announced it had begun work on its first well in the Yadavaran field in southwestern Iran operated by China's Sinopec. Yadavaran contains an estimated 12 billion barrels of oil, along with 12.5 trillion cubic feet of associated gas and 1.9 billion barrels of condensates.

It's one of five oil fields Iran shares with Iraq. The Islamic Republic produces around 68,000 bpd from the other four.

Iran is the second biggest producer in the Organization of Exporting Countries with 3.5 million bpd, 2.5 million of which is exported, earning around $100 billion a year.

It wants to boost output to 5.2 million bpd by 2015, and developing the shared fields, along with the giant gas South Pars gas field it shares with Qatar in the Persian Gulf, is intended to speed up that process.

Iranian Oil Minister Rostam Qasemi said in August 2011 the oil industry needs some $50 billion in investment to achieve that objective, which the Western-driven sanctions are aimed at thwarting by scaring off foreign investors.