WASHINGTON - A precipitous drop in Iran's currency, the rial, reflects relentless international pressure on the country over its suspected nuclear weapons program, the US State Department said on Monday.

International sanctions are cutting "deeper and deeper" into the Iranian economy, and Tehran needs to change its calculus on its nuclear program, State Department spokeswoman Victoria Nuland told reporters.

The rial plunged against the US dollar in open-market trading on Monday, and has lost more than a quarter of its value over the past week.

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