"Mexico is at the crossroads of the green economy politically, physically and practically," said UNEP Executive Director Achim Steiner at a news conference with Mexican President Felipe Calderon.

Steiner underlined the north American country's significant environmental challenges, "from high air pollution in cities and dependence on fossil fuels to land degradation and the need to fight poverty.

"But Mexico is also emerging as one among a group of developing economies who are bringing much needed leadership to the need for a new, comprehensive and decisive climate treaty," he said.

In four years Mexico has become second only to Brazil in terms of wind, solar, and biogas projects in the region, he said, underlining its growing participation in carbon markets.

The UN-sponsored World Environment Day began 36 years ago and takes place annually on June 5.

Next year's event will focus on combating climate change, one of the top priorities of UN Secretary General Ban Ki-moon.