(Reuters) - Iran's monthly revenues from oil sales have dropped 58 percent since just before the United States imposed harsher sanctions on the country in 2011 because of Tehran's disputed nuclear program, a senior U.S. official said.

The OPEC member's monthly crude oil revenues averaged an estimated $3.4 billion in the first half of this year, down from $6.3 billion in the year ago period, and $8 billion from the first half of 2011, said the source, who did not want to be named due to the sensitive nature of policy on sanctions.

Washington warned in late 2011 Iran's oil consumer countries that their banks would face being cut off from the U.S. financial system unless they significantly reduced purchases of Iranian oil. The EU has also cracked down on Iranian crude exports.

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