Iran cannot access close to 50 percent of its oil revenues, according to a report by a senior U.S. official provided to the Associated Press.

The report, published on August 30, estimates that the heavy sanctions imposed on Iran by the U.S. government have seriously affected Iran’s economy.

The estimate claims that more than $1.5 billion of Iran’s monthly oil revenues have to be held outside of Iran. The country’s gross monthly oil revenues are estimated at $3.4 billion.

In February, the U.S. intensified sanctions on Iran, impeding the country’s access to its oil revenues by requiring oil importers to put their payments for Iranian crude into locked bank accounts, which Iran can access only for purchasing non-sanctioned goods in the purchasing country.