Brian Hook, the U.S. special envoy for Iran, says Tehran has spent some $16 billion to fund Shi'ite-led militias in Syria and Iraq.
Hook made the comments in an interview broadcast on October 31 by Saudi broadcaster Al Arabiya but did not specify what period of time he was citing.
Shi'ite Iran has wielded influence in Iraq since dictator Saddam Hussein's ouster following a 2003 U.S.-led invasion, and it backs powerful Shi'ite militias in the country, including Iraq's Popular Mobilization Forces, often in conflict with the country's Sunni minority.
Washington still provides Baghdad with military, political, and financial aid, and Iraq has attempted to balance its ties to both the United States and neighboring Iran.
In Syria, Iran � along with Russia -- has provided crucial support to President Bashar al-Assad in his long battle with anti-government rebels and has supported Shi'ite militia operating under the Iranian Quds Force, the Islamic Revolutionary Guards Corps' (IRGC) foreign arm.
Iran also backs the Hizballah movement in neighboring Lebanon.
Israeli leaders have accused Iran of attempting to station advanced weaponry in Iraq and Syria that could be used to target Israel.
Hook said in the interview that Washington would continue to impose sanctions on Tehran, asserting they were a result of "destabilizing behavior in the region."
He also said the United States wants Iraq to reduce its economic dependency on Iran.
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