HANGZHOU, China, Sept. 4 (Xinhua) -- U.N. Secretary-General Ban Ki-moon on Sunday spoke highly of the ratification of the landmark Paris climate change agreement by China and the United States, calling for accelerated efforts from other countries to join the ambitious pact.

"With China and the United States making this historic step, we now have 26 parties in the UNFCCC (U.N. Framework Convention on Climate Change Convention) and 39 percent of global greenhouse gas emissions accounted for," Ban said at a press conference on the sidelines of the 11th Group of 20 (G20) summit in this eastern Chinese city.

Describing the progress as a major success, Ban praised the "outstanding leadership" demonstrated by Chinese President Xi Jinping and U.S. President Barack Obama, who handed over legal instruments to Ban on Saturday in a testament to their resolve to jointly tackle the global challenge.

Although the Paris deal was signed in the French capital in December, it will legally enter into force only after at least 55 countries accounting for at least 55 percent of global greenhouse gas emissions join the accord.

The U.N. chief urged other leaders to show leadership by accelerating their domestic ratification processes, "so we can turn the aspiration of Paris into the transformative climate actions the world so urgently needs."

Noting that the Hangzhou summit is "encouraging the speedy entry into force of this key agreement," Ban stressed that the international community needs to "keep this momentum."

In addition, Ban praised China's presidency of the G20 -- the premier platform for international economic cooperation -- as well as its focus on sustainable development.

He said he was delighted to see that for the first time in the G20 history, sustainable development is high on the agenda and will be highlighted in the outcome documents.