The Iranian rial slumped on Saturday, Sept 29, to a historic low against the U.S. dollar, Reuters reported citing Iranian media and currency tracking websites.

The rial fell by 3.6 percent and was trading at about 27,900 per dollar, compared with about 26,920 on Thursday and about 26,200 on Wednesday, according to Persian-language currency tracking website Mazanex. Another financial site, Mesghal.ir, had the rial at about 27,650 to the dollar on Saturday.

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 the U.S. and its allies of being aimed at obtaining a nuclear weapon. Iran denies this, but has faced successive rounds of stricter sanctions in the last two years.