TEHRAN — Iran’s president admitted Tuesday that the American-led economic sanctions on the country were partly to blame for a breathtaking 40 percent fall in value of the Iranian currency, the rial, over the past week. He pleaded with Iranians not to exchange their money for dollars and other foreign currencies.

Speaking during a news conference broadcast live by several domestic and international Iranian news channels, the president, Mahmoud Ahmadinejad, said Iran was facing a “psychological war” waged by the United States and aided by what he described as internal enemies.

He said the currency’s fall was caused in part by the sanctions imposed by the West over Iran’s disputed nuclear program, which have prevented it from selling oil and transferring money. He also blamed a domestic band of “22 people in three separate circles” who with “one phone call” could manipulate foreign exchange trades in Iran.

One Web site, Mashregh News, reported Tuesday that Mr. Ahmadinejad had ordered the arrests of those “disturbing the currency market.”