The Iranian currency slumped on Saturday to an historic low against the U.S. dollar, according to Iranian media and currency-tracking websites.

The rial fell by 5% and was trading at about 28,400 per dollar, compared with about 26,920 on Thursday and about 26,400 on Wednesday, according to Persian-language currency-tracking website Mazanex.

There was no clear reason for the latest slide in the value of the rial, which is traded in an informal market of money changers, where ordinary Iranians go to access hard currencies.

Previous falls have coincided with new sanctions or diplomatic isolation of Iran over its contested nuclear program, suspected by Israel, the United States and its allies of being aimed at obtaining a nuclear weapon. Iranians rushed to money changers to convert their savings into dollars, depressing the value of the rial.

The rial has seen a steep loss in value against the dollar in the last year. In June 2011, it was trading at about 12,000 to the dollar, or more than twice its value now, according to Mesghal.