A major construction project in wetlands seen as one of the "green lungs" of smog-choked Mexico City has raised concerns for the future of the UNESCO World Heritage Site.

A more than 1.7-kilometer (one mile) long reinforced concrete bridge is being built on a freshwater lake system that helps to regulate the temperature and prevent flooding in the city of nine million people.

Hundreds of trees have been cut down in the Xochimilco reserve, which is home to endemic species including the critically endangered axolotl, a salamander-like amphibian.

Residents and experts worry that the project is just the first step in an accelerated development of the area, which is listed as a Wetlands of International Importance under an intergovernmental conservation treaty.

"We're going to keep fighting, because we fear that they will want to continue to develop and fill the lakes here to make a commercial plaza," said Maria Alvarado, 60, a member of a group campaigning against the project.

Xochimilco is a magnet for tourists who ride colorful gondolas through its maze of canals and artificial islands created centuries ago by the area's indigenous peoples.

The splash of green in a vast urban sprawl has already been divided by one of the main highways skirting the city.

Activists filed several legal challenges and held demonstrations against the new bridge, which is being built at a cost of about $40 million and is almost completed.

"It's not just Xochimilco that loses -- the whole city loses," said Elia Solares, 65, a member of the campaign group.

'Enormous damage'

Mexico City Mayor Claudia Sheinbaum has defended the project, saying it will help traffic to flow more smoothly.

"It will be very important for the area ... both for public and private transport," she said last month, assuring residents that there was no ecological damage.

Her government has committed to restoring the wetland by planting endemic vegetation and connecting it with other lake areas.

"The wetland in this area of Xochimilco is going to be very important," she said.

But Luis Zambrano, a researcher at the National Autonomous University of Mexico (UNAM), fears the bridge will wreak destruction in a vital ecosystem.

"They're doing enormous damage," he said, warning that the project set a dangerous precedent.

"They're opening the door to the destruction of any ecosystem, no matter how protected it may be," he said.

Alvarado and her fellow campaigners plan to keep up their struggle to stop the bridge ever being used.

"We will not give up our fight," she said.

"Even if this bridge is already working, we will keep fighting against it being used."