Secretary-General Ban Ki-moon is urging world leaders to sign the landmark agreement to tackle climate change reached in Paris in December at a ceremony at U.N. headquarters on April 22.

The U.N. chief told the world body's 193 member nations Wednesday that the ceremony will be a key step toward ratification of the agreement. Every U.N. treaty is first signed and then ratified.

The climate agreement needs to be ratified by 55 countries, representing at least 55 percent of greenhouse gas emissions, to enter into force.

Ban said he has issued invitations and the leaders of Peru, France and Morocco, who hosted the last three climate conferences, will attend.

The U.N. chief said the participation of all leaders will keep the global spotlight on climate change.