It's time for the United States to hold Iran accountable.

President Hassan Rouhani of Iran accused the United States on Thursday of not complying with the landmark nuclear agreement that took effect in January, and said American credibility would suffer if the accord were not honored.

In his United Nations General Assembly speech and later at an hourlong news conference, Mr. Rouhani criticized what he described as an American failure to adhere to obligations under the agreement, which relaxed many economic sanctions on Iran in exchange for verifiable pledges of peaceful nuclear work.

Mr. Rouhani told reporters that despite the intense diplomacy that had achieved the agreement between Iran and big powers including the United States, there was “not a very stellar report card by the United States when it comes to fulfilling the agreement.”

Nonetheless, Mr. Rouhani’s criticism appeared relatively muted and he seemed to carefully avoid any indication that the agreement was at risk.

He spoke a day after American export licenses were granted so that Iran could buy dozens of new planes from Boeing and Airbus, the biggest commercial transactions between Iran and the West in decades and a direct outcome of the nuclear agreement.

Mr. Rouhani called the agreement a model for how to resolve disputes peacefully. But he coupled the praise with a warning against “illegal actions” by the United States.

He complained that American restrictions on banking and dollar transactions with Iran, which were unaffected by the nuclear agreement, have discouraged many banks from engaging with the country because they worry about possible penalties from the United States.

“They’re frightened, the big banks,” he said at the news conference, accusing the Americans of “creating a lot of doubt.”

Mr. Rouhani also was highly critical of a Supreme Court ruling in April— which had nothing to do with the nuclear accord — permitting the United States to impound nearly $2 billion in Iranian central bank assets. That money would compensate American victims of overseas terrorist attacks in 1983 and 1996 that the United States says were sponsored by Iran.

Impounding these assets, Mr. Rouhani told reporters, was “an international heist, to be quite frank with you.”

Facing re-election next year, Mr. Rouhani is under enormous domestic political pressure to show the beneficial consequences of the nuclear agreement.

He said that “any failure on the part of the United States in implementing it would constitute an international wrongful act and would be objected to by the international community.”

While the lifting of nuclear sanctions has enabled Iran to produce and sell far more oil, and its economy is growing faster than in other oil-exporting countries, the nuclear accord has not led to a trade and investment boom in Iran as Mr. Rouhani had forecast.

Hard-line anti-American elements of Iran’s hierarchy have also been highly critical of the nuclear deal, and stand to benefit politically at Mr. Rouhani’s expense should it fail.

Mr. Rouhani, who has advocated improved relations with the United States and other Western powers, said nothing about his own political prospects. But he said any failure in implementing the nuclear agreement “will further erode the credibility of the United States in the world.”

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