TEHRAN — Iran, squeezed by punishing American sanctions, is confronting its most severe economic challenge in 40 years, President Hassan Rouhani said on Wednesday, arguing that the Iranian government “should not be blamed” for the crisis.

The Trump administration reimposed sanctions on Iran last year after President Trump decided to withdraw from the international nuclear accord reached with Tehran in 2015, calling it a “horrible one-sided deal.” Under the pact, Iran agreed to essentially freeze its nuclear program for at least 15 years in exchange for relief from oil and financial sanctions.

Under American pressure, dozens of European companies have abandoned operations in Iran that they had started after the signing of the nuclear agreement, leaving thousands of Iranians jobless. Reimposed banking sanctions have sharply curtailed foreign investment and access to international credit, and oil sanctions have more than halved Iran’s crude exports, its main source of income.

“Today the country is facing the biggest pressure and economic sanctions in the past 40 years,” since the 1979 Islamic Revolution, Mr. Rouhani said in a speech commemorating the anniversary that was broadcast on state television.