The U.S. reached the estimates by looking at Iran's trade imbalances with oil importers based on customs data from each of the relevant countries. The figures show Iran cannot spend the full amount it earns because it is limited to buying only non-sanctioned goods for imports from the small pool of trading partners. And it is not able to repatriate the money to fill its foreign reserve coffers or cover any budget shortfalls.
Garbis Iradian of the Institute of International Finance, an economic think tank, noted that despite wave after wave of sanctions, Iran continues to run a trade surplus. But that surplus has been shrinking steadily since 2011. The assets piling up abroad could render most of that remaining surplus essentially unusable.
"This is a major development," Iradian said. "If they don't have access to this, it is an additional burden, and if that continues on they will feel the pain," he added. "It seems the sanctions intensified with this accessibility issue."
Iradian, the deputy director of the IIF's Africa and Middle East Department, said Iran's total trade surplus has fallen from about $70 billion in 2011 to about $44 billion in 2012. The IIF estimates it will reach about $38 billion by the end of this year. And with $1.5 billion a month accumulating in restricted accounts, some $15 billion of the $38 billion surplus may be out of reach.
"This brings down their trade surplus to almost zero," said Iradian. "That is quite severe. ... They are entering a dangerous zone."
The senior U.S. official also disclosed a previously unreleased government estimate that crude oil revenues have fallen 58 percent since late 2011 because of sanctions. The revenues averaged an estimated $8 billion a month in the first half of 2011, then fell to $6.3 billion in the first half of 2012 and an estimated $3.4 billion monthly in the first half of 2013, according to the assessment.