Building new coal-fired power plants would be "detrimental" to efforts to curb carbon emissions that currently place the world on a path to warm 2.9-3.4 degrees this century, a United Nations scientist says.

Jacqueline McGlade, chief scientist for the UN Environment Program, said new coal plants would be "entirely contradictory" to the need to close a wide gap between national promises made at last year's Paris climate summit and the carbon emissions trajectory.

"Anything that leads to an increase in greenhouse gases we see as detrimental," Professor McGlade said. "From an environmental perspective it's just a no go [for new coal]."

The comments come as the Paris climate agreement – signed by almost 200 nations pledging to keep global warming to less than 2 degrees – comes into force on Friday.