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A top Iranian environmental official, who symbolised government efforts to encourage expat Iranians to return home, has quit just seven months into his job, sources said on Monday.

The resignation of Kaveh Madani, who was a young professor at London's Imperial College before returning to Iran in September 2017, followed reports of mounting conservative pressure against him.

The government of President Hassan Rouhani, which has called on Iranians abroad to come home and help develop the country, appointed the water conservation expert as deputy head of the country's environment department.

But in recent weeks, conservative outlets published photos purporting to show him drinking and dancing on holiday. Hardliners called him "debauched" and demanded he be sacked.

Shortly after arriving in Iran last year, Madani had told the English-language Tehran Times that many Iranians abroad were "waiting and watching closely to see what's going to happen".

"If I succeed, we might see more people coming back to help the government," he said.

But on Monday a colleague of Madani confirmed reports that he had resigned while out of the country.

In February, Madani was briefly detained by one of Iran's security agencies amid a crackdown on environmentalists that saw several activists arrested on espionage charges.

One, Kavous Seyed Emami, died in Tehran's Evin prison after being accused of spying for the United States and Israel. Iranian authorities said he had committed suicide in his cell, but this has been disputed by the family.

Madani studied in Tabriz in northern Iran before getting his PhD in civil and environmental engineering from the University of California, according to Imperial College's website.

"I'm the representative of a generation who left the country and the whole country is complaining about losing this generation and brain drain," he told the Tehran Times in December.

He also tweeted at the time: "I have returned with the hope of creating #hope".

After hearing of Madani's resignation, reformist lawmaker Mahmoud Sadeghi criticised authorities for the message he said they were sending to "elite Iranians living abroad".

The water expert's departure comes as Iran suffers from years of drought, which has devastated its agriculture.

There have been reports of persistant protests by farmers in Isfahan in recent weeks, which have been denounced by conservatives including the city's Friday prayers leader.