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The sentence against Hekmati comes amid heightened U.S.-Iranian tensions.

The Obama administration has approved new sanctions against Iran over its disputed nuclear enrichment program, specifically targeting the regime’s central bank and its ability to sell petroleum abroad, but the stronger penalties have not taken effect. Iran has responded with warnings to American vessels against entering the Strait of Hormuz, the strategic waterway that carries to market much of the oil pumped in the Middle East.

Mr. Obama approved the new penalties against Iran on New Year’s Eve, despite his administration’s fears they could lead to a spike in global oil prices or cause economic hardship on American allies in Europe and Asia that are still importing petroleum from Iran.

The measures affect foreign financial institutions doing business with Iran’s central bank by barring them from doing business in the United States. They would apply to foreign central banks as well for transactions related to petroleum.

But the sanctions won’t take effect for six months. The president also can waive them for national security reasons or if the country in question significantly reduces its purchases of Iranian oil. The State Department says it is trying to implement the law in a way that maximizes pressure on Tehran while causing minimal disruption to the U.S. and its allies.

American officials are concerned that Hekmati’s case may become a political tool for the Iranian government.

Having imposed the worst possible sentence immediately, Iran could now seek to drag the case out. In past cases Iran has held out the possibility of releasing American prisoners on humanitarian grounds, presumably in the hopes of gaining a counterconcession from Washington. September’s release of a pair of American hikers held captive by Iran for two years is the most recent example.

Associated Press writers Lolita C. Baldor, Anne Gearan and Julie Pace contributed to this report.