Iranian authorities on Sunday announced they were raising the price of petrol for heavier consumers, as the United States steps up sanctions seeking to isolate the country’s economy.

Iran’s quota system prices petrol in three tiers – 1,500 tomans per liter for a consumer’s first 60 liters in a given month, 3,000 tomans per liter for their next 50, and 5,000 tomans per liter after that. The toman is an informal unit equivalent to 10 Iranian rials.

On Sunday, government spokeswoman Fatemeh Mohajerani said the price for the third tier would double to 10,000 tomans (about 4.5 US cents).

The rate will increase to 10,000 toman rials from the morning of September 8,” Mohajerani said in remarks carried by state media.

“Different numbers were mentioned in expert meetings, but because the president had promised the people, the rate of 10,000 tomans was set,” she added.

She later clarified that the price of the first and second tiers would not be changed.

Iran, a major oil producer with some of the world’s cheapest fuel, has for years heavily subsidized petrol, making any changes to pricing politically sensitive.

The development comes as the Iranian currency was trading on the black market Sunday at more than 2.2 million rials to the US dollar, according to websites that track the rate.