Jassim al-Assadi, 65, head of the environmental group Nature Iraq, has appeared regularly in local and foreign media to raise awareness of the threats facing the country's southern wetlands, a UNESCO World Heritage Site imperilled by years of drought.

Assadi was "driving on the highway from Hilla to Baghdad" on Wednesday morning when he was abducted, his brother Nazim told AFP.

"Five kilometres (three miles) before the capital, two cars stopped him, and armed men in plain clothes handcuffed him and put him onto one of the vehicles, taking him to an unknown location," he said.

"My cousin was with him," Nazim Assadi added. "They left him on the road."

The family has not heard from Assadi since and police are investigating, the brother said, adding that the kidnappers had not contacted any relatives.

"We need time to understand the reason. The motive isn't clear for now," Nazim Assadi said.

An Iraqi security official, requesting anonymity as they were not authorised to speak to the media, confirmed to AFP that the family had reported Assadi missing.

While Iraq has regained relative stability after decades of conflict and unrest, assassinations and kidnappings of activists and officials remain common.

Iraqi civil society groups have decried the continued presence of armed factions across the country and the proliferation of weapons, while tribal conflicts often lead to deadly violence.

Since 2006 Assadi, a hydraulic engineer by profession, has been involved in numerous initiatives to restore the marshes, where he was born.

The area had faced destruction at the hands of dictator Saddam Hussein, and has been further jeopardised by climate change.

After years of drought, winter rains this year have brought some respite to herders and livestock in the famous wetlands, which were parched and dusty last summer.

The United Nations says Iraq is one of five countries most exposed to some impacts of climate change.

The marshes have also been hit by reduced flows from the Tigris and Euphrates due to dams built on some rivers and tributaries in neighbouring Turkey and Iran.