In his first department-wide policy guidance statement since taking office a year ago, he told his 70,000 staff: "The environment has been one of the central causes of my life."

"Protecting our environment and meeting the challenge of global climate change is a critical mission for me as our country's top diplomat," Kerry said in the letter issued on Friday to all 275 US embassies and across the State Department.

"It's also a critical mission for all of you: our brave men and women on the frontlines of direct diplomacy," he added in the document seen by AFP.

He urged all "chiefs of mission to make climate change a priority for all relevant personnel and to promote concerted action at posts and in host countries to address this problem."

The clarion call comes ahead of key UN-led talks in Paris next year when the international community is due to try to set new emissions goals for greenhouses blamed for global warming.

The emission levels will be applicable to all countries, not just the developed world, and will come into effect in 2020.

The new agreement will replace the Kyoto treaty which is due to expire in 2015.