By day’s end, Iran’s Fars News Agency was reporting that it cost 15,150 rials to buy one dollar in Tehran, compared with 13,400 rials to the dollar a few days earlier. In late October, it cost about 12,500 rials to buy one dollar in the capital city, which means the Iranian currency has plunged in value by about 20 percent against the dollar in the past few months.

The currency crisis could hardly come at a worse time for Iran, which has been pummeled economically by the cumulative effects of four rounds of United Nations Security Council sanctions, as well as sanctions imposed by the United States, Canada and the European Union over the country’s suspect nuclear energy program.

An increasing number of international companies are no longer willing to engage in new business with Iran; the European Union has talked about imposing a boycott on Iran’s oil, its major export; and the Obama administration is expected to enact a harsh law soon that could severely penalize banks in friendly countries that do business with Iran’s Central Bank. Officials in Iran have spoken more frankly in recent days about the destructive impact of Iran’s isolation.

The United States Treasury Department added to the pain on Tuesday, putting 10 companies based in Malta on a blacklist for acting as front companies for the Islamic Republic of Iran Shipping Lines, which has already been blacklisted. The Treasury said in statement that it took the action because the Iranian shipping line and its subsidiaries “have increasingly relied upon multiple front companies and agents to overcome the impact of U.S. and international sanctions and increased scrutiny of their behavior.”

The rial’s loss in value partly reflects the impact of a stubborn inflation problem in Iran, caused in part by the government’s phasing out of subsidies for fuel and other staples that has caused prices to rise sharply. To cushion the impact on Iranian consumers, the government distributed cash payments, which in some ways worsened the inflation problem by increasing the supply of rials in Iran. Iran’s official news media have said the inflation rate was about 20 percent, although outside estimates were higher.