BERLIN—A year after German engineering giant Siemens AG pledged to retreat from Iran under international pressure, it is grappling with a thorny problem: a big jump in revenue in the Islamic republic.

Siemens has kept a promise not to pursue new projects in Iran. But its existing contracts there underscore how international efforts to curb Tehran's nuclear ambitions have had only limited impact on the state's ability to draw on the technology and expertise it needs to maintain its broader infrastructure.

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