Five environmentalists have been charged in Iran with national security crimes punishable by death, in a development the UN environment head said was deeply troubling.

The activists from the Persian Wildlife Heritage Foundation were arrested in January alongside at least four other people, and face allegations of spying, which human rights campaigners and Iran’s own government say are unfounded.

Concerns about environmental challenges, including water shortages, were a trigger for nationwide protests that began early this year before taking on a wider political dimension. They came as Iran faces its worst drought in modern times.

The impasse has undermined the administration of Hassan Rouhani, which has remained adamant that there is no evidence of wrongdoing against the environmentalists. By keeping them in jail and charging with such serious crimes, Rouhani’s hardline opponents are flexing muscles in a show of strength about their upper hand on security matters.

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In January, at least nine activists were detained in mass arrests of environmentalists carried out by the Revolutionary Guards, the elite forces that act independently of Rouhani’s government and have huge influence within the country’s conservative-dominated judiciary.

One of the detainees, Kavous Seyed-Emami, a renowned Canadian-Iranian environmentalist, died in a prison in Tehran under mysterious circumstances. The authorities said he killed himself, but that claim has been met with widespread scepticism.

At around the same time, Kaveh Madani, deputy head of Iran’s environmental protection organisation, was detained for 72 hours before eventually leaving the country to live in exile. But at least eight other environmentalists remain in jail.

Facebook Twitter Pinterest Kavous Seyed-Emami. The UN says it has been kept in dark about his death in custody. Photograph: AFP/Getty

The New York-based Center for Human Rights in Iran cited an Iranian lawyer as saying that five of them have had their initial charge of espionage elevated to “corruption on earth” – the maximum penalty for which is execution.

The UN Environment head, Erik Solheim, told the Guardian that the UN is deeply troubled by the cases. “I have transmitted our concerns twice in writing to the Iranian authorities, and have also spoken with the authorities in Tehran,” he said. “But this latest news gives us even greater cause for alarm.”

Solheim said the UN has been kept in dark about the death in custody of Seyed-Emami as well as the new charges brought against the environmentalists. “This sends an extremely ominous message to Iran’s environmentalist community who are striving to protect the Islamic Republic’s unique environment,” he added.

“The message that needs to be sent is that environmentalists, like the dedicated people who are under arrest, deserve the utmost support and fullest protection which Iran’s laws and constitution guarantee. At present, Iran’s environment is under immense pressure – with water stress, land degradation, air pollution, loss of wildlife and sand and dust storms. All who seek to protect it must be supported. All hands count.”

It is still unclear why the authorities have been suspicious of the Persian Wildlife Heritage Foundation (PWHF), which was founded by Seyed-Emami. The judiciary has accused the professor of being a CIA-Mossad agent, saying his organisation had used surveys of endangered Asiatic cheetahs as a pretext for spying in strategically sensitive areas. A reporter close to the Revolutionary Guards claimed recently that the environmentalists were planting cameras and collecting soil samples to identify sensitive areas used by Iranian military for missile tests.

Officials have failed to release any evidence in public to substantiate their claims.

The five environmentalists are Houman Jowkar, Taher Ghadirian, Morad Tahbaz, Sepideh Kashani and Niloufar Bayani. Tahbaz, who is Iranian-American, was a CEO of the group. Bayani has a history of working with the UN, including for projects in Afghanistan. She returned to Iran, her birthplace, last June to join the PWHF.

One source familiar with Iran’s political machinations said its intelligence apparatus has sensed weakness in dealing with growing environmental challenges, which it views as a national security issue. The source said officials are suspicious of NGOs with great international exposure that have dual nationals involved.

“After the death in custody of Seyed-Emami, the Revolutionary Guards went on defensive and to extremes to prove that something was wrong, especially when the government is saying they haven’t done anything wrong,” the source said.

The three other environmentalists who have not been charged with corruption on earth, but whose fate are unclear are Amir Hossein Khaleghi, Sam Rajabi and Abdolreza Kouhpayeh.