"The sanctions will force a choice between buying Iranian oil or engaging in the US financial system, the largest in the world, " said Brian Katulis of the Center for American Progress.

BBC: Iran's rial slides against US dollar on fresh sanctions

As the world financial system careened toward debacle and then new scrutiny in recent years, unregulated oil and metals traders were at work in relative obscurity in Switzerland building massive businesses, capitalizing on the rise of commodities prices and China.

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