French Environment Minister Segolene Royal said Wednesday she would head the UN's COP21 climate forum after a long-time political rival, former foreign minister Laurent Fabius, stepped down from that role.

French President Francois Hollande "offered me the opportunity yesterday to take on this responsibility, and I accepted," she told French television news channel iTELE.

A spokesman for Hollande's office confirmed the appointment but said a date had not been set.

COP21 is the acronym for the 21st conference of parties to the UN climate arena.

Its president comes from the country who hosts the forum's annual high-level gathering, and usually plays a key role in brokering agreements in the complex negotiations.

Fabius, appointed in November, was hailed for shepherding the 195 nations to a historic deal in Paris that involves curbing carbon emissions and channelling hundreds of billions of dollars to poor countries exposed to climate change.

He was pressured to relinquish the job on Monday after he stepped down as foreign minister and was named to head France's constitutional court.

Critics argued that he could not simultaneously be on the court, which is supposed to be above the political fray, and also in charge of the COP, where politics and powerful interests collide.

Royal, Hollande's former companion and the mother of their four children, was among the first to raise objections, calling for "the rules to be clarified" on double postings.

In her new role, Royal will guide the first steps for implementing the December agreement, which set the ambitious goal of capping global warming at "well under two degrees Celsius" (3.6 degrees Fahrenheit) above pre-industrial levels.

The 32-page deal also calls on rich nations to muster at least 100 billion dollars (90 billion euros) a year in climate aid from 2020. Just how that will happen has yet to be worked out.

"We can't let the momentum slip," Royal told AFP. "There's a lot to do. I have already started."

Among her priorities, she said, are putting a price on carbon and climate action in Africa.

"Africa can be a laboratory for transitioning away from fossil fuels using solar, geothermal, hydroelectricity," she said.

"We need a systematic approach, and we have to get moving before the end of 2016."

Royal will also seek to highlight a signing ceremony for the agreement on April 22 at the United Nations in New York. She has said Hollande will attend and encourage other heads of state to join him.

France will hands on the climate baton to Morocco, which will organise COP22 in Marrakesh from November 7 to 18.