Iran’s new president, Masoud Pezeshkian, ran as a reformist candidate. But when he announced on Sunday that his cabinet nominees included several conservatives and only one woman, he faced a fierce backlash, with a high-profile vice president resigning and political allies accusing him of abandoning campaign promises to bring change.

The resignation on Sunday of Mohammad Javad Zarif, who had been appointed vice president for strategy and had led a search committee for cabinet nominations, shocked Iran’s political circles. He had been a prominent face of Mr. Pezeshkian’s campaign, traveling across the country and telling voters to give change a chance. Now, he was abandoning the government in a public display of deep divisions before it was even formed.

In a post on his Instagram account, Mr. Zarif said it had become clear to him that he could not bring about the kind of domestic change that people had expected.

“I am not satisfied with the outcome of my work, and I’m ashamed that I could not adequately achieve what I had promised about representation of women, youth and ethnicities and the expert opinion of the committees,” he said.

Mr. Zarif followed up on Monday with another post on to Instagram, saying that his resignation did not mean he regretted supporting Mr. Pezeshkian or that he had lost hope in the new government, but rather “it means that I doubt whether I can be effective as a vice president of strategy.”

More stunning was the timing of Mr. Zarif’s resignation. As Iran’s former foreign minister, its most seasoned foreign policy expert and the top negotiator in the 2015 nuclear deal between Iran and world powers, he was bowing out as a possible regional war loomed.