US Secretary of State John Kerry has called on American ambassadors around the world to make the fight against climate change a top priority ahead of new UN talks next year.

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.

The United States, which along with China is the world's top polluter, did not ratify the Kyoto Treaty arguing that developing countries should also be obligated to cut greenhouse gases.

Kerry travelled to China last month and won agreement from Beijing that the US and China would cooperate closely ahead of the Paris talks as they aim to agree emissions targets.

In his policy guidance, Kerry set out a seven step program to enhance the focus on tackling climate change including boosting multilateral and bilateral efforts.

"We're talking about the future of our earth and of humanity. We need to elevate the environment in everything we do," he said.

It was, he said "our call to conscience as citizens of this fragile planet we inhabit."

Kerry is currently weighing a decision on whether the US should approve the building of a pipeline carrying oil from the tar sands of Alberta, in Canada, to US refineries in Nebraska and then Texas.

An environmental review found that the Keystone XL project would not add significantly to greenhouse gas emissions.

But opponents are calling on Kerry to reject the proposals by Canadian operators TransCanada, and a small but noisy protest was held late Thursday outside the State Department.