Michel Forst voiced alarm over the alleged "persecution, penalisation and harassment" of Mammadli, who has now been in pre-trial detention for six months on "allegedly trumped-up criminal charges" in what the rapporteur called "apparent reprisal for his environmental activism".

"I am gravely concerned about the deterioration in the treatment of Mr Mammadli and the crackdowns on civil society actors, including environmental defenders", Forst said in a statement.

Mammadli risks up to eight years behind bars on smuggling charges which human rights groups claim are bogus.

He and activist Bashir Suleymanli had formed a civil society group called Climate of Justice Initiative, which set out to promote environmental justice in the tightly-controlled, oil-rich nation.

The COP29 UN climate summit is being held in Azerbaijan's capital Baku from November 11-22.

Forst said it appeared that the charges and the lengthy detention "were a form of retaliation against Mr. Mammadli for his efforts in the lead up to COP29".

His ongoing detention on charges subject only to a preliminary investigation "therefore appears grossly unreasonable and disproportionate".

"The length of Mr. Mammadli's pre-trial detention also strongly indicates that it is a measure that is punitive in nature, aiming to intimidate Mr. Mammadli and other environmental defenders from speaking out, particularly in the lead up to COP29. This is unacceptable."

Mammadli's health has deteriorated in detention and served to "further penalise" him, Forst said, calling for his immediate release and for the charges to be "immediately dropped".

As UN special rapporteur, Forst is tasked with taking measures to protect any person experiencing or at imminent threat of penalisation, persecution or harassment for seeking to exercise their rights under the convention.

Azerbaijan is a party to the convention.

The former Soviet republic wedged between Russia and Iran has faced considerable scrutiny over its hosting of COP29.

Last week the European Parliament denounced Azerbaijan's crackdown on critics and said its "ongoing human rights abuses are incompatible" with hosting the summit.